The barrier was never completely impenetrable and there is little doubt that in early Christian Ireland there was a marked cultural unity, at least at an upper class level. It is worth pointing out, however, that when the island was moving towards political unity in the eleventh century – that is from the reign of Brian Boru onwards – the High Kings ruled co fresabra, that is 'with opposition' and that opposition generally came from Ulster. Until the seventeenth century Ulster remained the most Gaelic of the four provinces.

Defeated on more than one occasion on their own element, the sea, the Vikings had only a couple of toe holds in the north and concentrated their town building enterprise in Leinster and Munster. Except for the mountains and the bogs, the Anglo-Normans overwhelmed Leinster, Munster and Connacht but in spite of the efforts of John de Courcy, Hugh de Lacy, King John and the Red Earl, Ulster was always a dangerous marchland.

Never more than the coastlands of Antrim and Down were held for long and by the fifteenth century the Clandeboye O'Neills, thrusting eastwards, had left the English Crown hanging on by its eyebrows at Carrickfergus, Strangford and Portaferry. The descendants of these Norman settlers – with surnames such as Fitzsimons, Savage, Fleming and Russell – are today mostly Catholic living in Lecale or the tip of the Ards peninsula. Officials in Dublin Castle routinely referred to the north-west of Ireland as the 'Great Irishry'.

After Elizabeth had conquered Ireland from end to end, her successor, James I, implemented his ambitious scheme to settle Ulster by colonising most of the province with loyal British subjects early in the seventeenth century. Ulster was then the least anglicised part of Ireland, and so the Gaelic natives were confronted by alien planters adhering to a variety of Protestantism far distant from their own Catholicism.

Franciscans and priests returning from seminaries on the European mainland instilled a new zeal: in Ulster, therefore, the uncompromising spirit of the Counter-Reformation faced the inflexible determination of the Puritan and Presbyterian settlers. At the same time the Protestant planters felt duped and let down by the Crown: the land had not been cleared of natives as promised and the newcomers had constantly to defend themselves against kerne lurking in the woods – indeed, it was not until the census of 1871 that Protestants outnumbered Catholics in the province of Ulster.

Hostility, suspicion and uncertainty created a dangerously unstable atmosphere. Political instability in England – just as much as in Ireland – ensured that wounds were not given time to heal, and by the time that warring had cease in 1691, memory of dispossession, massacre, persecution and betrayal was deeply etched into the folk consciousness. Of course this was true of the whole island but the resentment and hatred were far more acrid in Ulster than in the rest of Ireland – still the north in this respect was not a place apart but a variation on a theme.

Later I will be restating the argument that Northern Ireland's problems bear an uncomfortable resemblance to those of eastern Europe. At this stage it's worth stating that I can find no evidence of ethnic hatred in Ireland until well into the sixteenth century. The Vikings blended in easily: King Magnus Barelegs of Norway was so called because he was so entranced with Ireland that he adopted Irish dress, some time before he got himself killed in Downpatrick. The mixed Norse-Irish were known as the Gall-Gael and their descendants survive with surnames such as MacManus, Searson, Harold and Doyle.

Only a few years after the Norman conquest had begun, Gerald of Wales was complaining that the newcomers had become 'more Irish than the Irish themselves'. Norman lords cheerfully intermarried with the native population and their descendants adopted Irish speech, dress, music and customs – those who had been de Burgos became Burkes, le Poer became Power, de la Rochefort became Roche, le Sauvage became Savage, and so on.

In contrast the Elizabethan – Jacobean conquest and colonisation were so traumatic and carried through at a time of bitter religious division across Europe that this time round the lines of division between natives were not smoothed over. That lack of blending was primarily in the folk memory. Historians and genealogists are now satisfied that a lot more blending of blood occurred than most people realise or are prepared to admit.

It is a mistake to believe all Protestants are descended from British colonists and that all Catholics are descended from the native Irish. In the east of Ulster, where colonisation was heavy, many native Irish became Protestants and dropped the 'Os' and 'Macs' from their surnames. In the west of the province settlers often came without their womenfolk and married local girls: within a generation or so many of their descendants were Catholics, speaking Irish.

Border Scots, fleeing from James I's suppression of the 'middle shires', were nominal Catholics but conformed rapidly to the Established Church to get land. Industrialisation, the Evangelical Revival and the Catholic Renewal produced further mixing in the nineteenth century. It's worth noting that Hume, Sands and Adams are lowland Scots names, and that the surnames of three prominent Unionist politicians – O'Neill, Maginnis and McCusker – are native Irish.

In the wake of the Home Rule Bills and the Gaelic Revival towards the end of the nineteenth century, there was a growing view that Ulster Protestants were teutonic in race, possessing the virtues of thrift, capacity for hard work and respect for law and order. At the same time nationalists accepted this Protestant assumption of racial separateness, for they were emphasising their Celtic origins and laying claim to inherent characteristics such as hospitality, passion and love of poetry.

By the beginning of the twentieth century, however, Ulster Protestants thought themselves to be Irish to a much greater extent than today. At the great Ulster Unionist Convention in Belfast's Botanic Gardens the entrance to the vast pavilion was decorated with painted shamrocks and harps and in large letters the words 'Erin go Bragh'. Ulster Unionist MPs at Westminster unhesitatingly considered themselves Irish. During the course of the twentieth century all that changed.

In 1914, after two years of crisis both the Liberal government and the Conservative opposition were quietly agreed that the only route out of the Irish bog was partition. Neither Asquith could persuade his Nationalist allies, nor Bonar Law convince his Ulster Unionist allies, to accept this solution.

In 1916 the Ulster Unionists changed their minds. At a crucial meeting of the Ulster Unionist Council a majority voted to accept the exclusion of the six north-eastern counties from the operation of a home Rule administration based in Dublin. It was a Unionist decision, not a government one, that Cavan, Donegal and Monaghan would be abandoned. According to Ronald McNeill MP: 'Men not prone to emotion shed tears'.

At the end of 1918 Unionists found themselves in a position vastly more favourable than they had been in on the eve of the Great War. Lloyd George was still Prime Minister but more than half the MPs at Westminster were Conservatives (still calling themselves Unionists), very sympathetic to northern Protestant aspirations. Abstentionist Sinn Fein had supplanted all but six Nationalist MPs – what Arthur Balfour described as 'the blessed refusal of Sinn Feiners to take the Oath of Allegiance in 1918' allowed Ulster Unionists to work out an arrangement to suit them, at leisure and with the minimum of interference.

That arrangement was the Better Government of Ireland Bill drawn up by a cabinet committee chaired by Walter Long, who just happened to be a former leader of the Irish Unionists. When the Bill became law in December 1920, some loyalists asserted they were making a 'supreme sacrifice' in accepting it. Read the debates, however, and it is clear that they got what they wanted. Captain Charles Craig, MP for South Antrim, said in the Commons that 'without a parliament of our own constant attacks would be made upon us, and constant attempts would be made…to draw us into a Dublin parliament'. In short he distrusted the future intentions of Labour and the Liberals. 'We believe that if either of those parties, or the two in combination, were once more in power our chances of remaining a part of the United Kingdom would be very small indeed.'

Had there been a strong nationalist representation at Westminster, it might have been very much more difficult to get all of the six north-eastern counties for Northern Ireland. There was no reason why units other than counties, such as Poor law Unions, could not have been used as a guide for drawing the frontier with the aid of the 1911 census map – the last for a very long time to show where Catholics and Protestants formed majorities.

The 1916 UUC decision not to ask for all nine counties still stood. As Craig put it: 'A couple of Members sick, or two or three members absent for some accidental reason, might in one evening hand over the entire Ulster parliament and the entire Ulster position.'

The frontier designated in the 1920 Government of Ireland act stood and still stands. After the Anglo-Irish War, the 1921 treaty would not have been signed but for article XII providing for a boundary commission. The article was not a bone of contention in the Dail Treaty debates, mainly because of a general assumption that great swathes of Northern Ireland would be assigned to the Free State.

In fact the article was full of holes, not discovered until after the Civil War was over and until the Boundary Commission got to work in November 1924: the commissioners were not independent; there was no provision for a referendum, as in Silesia and Schleswig; and the boundary was to be fixed not only 'in accordance with the wishes of the inhabitants' but also economic and geographic considerations had to be taken into consideration. Only minor alterations were recommended and, after the resignation of the Free State representative, Eoin MacNeill, the report was suppressed in December 1925.

The devolved administration in Belfast had no doubts that Northern Ireland was not a variation on a theme but a place apart. Decisions taken, not only in the Northern Ireland Parliament but also at Westminster and in Dail Eireann, certainly widened the gap between north and south until it became a gulf – a widening gulf, to use the title of Dennis Kennedy's book.

Both the Belfast and Dublin administrations governed with a philosophical basis which relied on a combination of amnesia and fantasy. The Cosgrave and de Valera governments worked to a monocultural agenda which involved a denial of the south's multicultural past. In the north the representatives of the majority had sought and obtained an arrangement in which they could all but forget the aspirations, allegiances and cultural concerns of one third of the citizens. When Craigavon said 'We here in Ulster' as he seems to have done in one out of every three of his speeches – he was not including the Catholic minority.

He and his successors were helped by the creation of a semi-theocratic state in the rest of the island, Dublin's territorial claim (which of course was always there before it was enshrined in the 1937 constitution, and by the failure of Westminster and Whitehall to keep an eye on the ball in Northern Ireland while at the same time refusing to allow discussion of the region's internal affairs in Parliament. On top of that Northern Ireland had a strategic value to Britain which became vital during the Second World War: it was during that conflict and in the years immediately after that Northern Ireland and Eire were most obviously two places apart.

In the post-war years the declaration of the Republic and the 1949 Ireland Act drew a new line of contrast, but so also did the creation of the welfare state. London underwrote the cost and the outcome was a more striking increase in living standards than at any other time in the century. The south was too inward-looking and, perhaps, too lacking in resources to follow suit. In 1951 Sean O Faolain observed that Ireland had been 'snoring gently behind the Green Curtain that we have been rigging up for the last thirty years – Thought-proof, World-proof, Life-proof.' In the same year John D'Alton, Archbishop of Armagh, described the welfare state as 'a milder form of totalitarianism'. Sir Wilfred Spender had earlier confided to his diary that it was de Valera's intention to keep the south 'as remote from the world's affairs as Mars'. The Mother and Child controversy showed that Costello and MacBride were equally resistant to modernising tendencies.

In the nineteenth century as Westminster made its leisurely progress towards adopting a democratic system of representation it became more acutely aware that it had a majority/minority problem. After the First World War the peacemakers in Paris created a range of states from the wreck of the Russian, Austro-Hungarian, German and Turkish empires. Of these only Austria had a fairly homogeneous ethnic composition and it has been estimated that more than 25 million found themselves as national minorities after 1919. Only two thirds of the inhabitants of Poland spoke Polish, for example, and there were 4.6 million Germans, Poles, Ruthenes and Magyars in Czechoslovakia out of a total population of 14.3 million, not to speak of differences between Czechs and Slovaks.

The creation of the border in 1920-21 largely corralled Ireland's majority/minority problem within Northern Ireland. The Protestants in the south formed a sufficiently small minority for their existence not to dominate the affairs of state. Those put at the greatest disadvantage were Protestant farmers in Donegal, Cavan and Monaghan, and (to a lesser extent) in West Cork, Wicklow and Wexford. A very high proportion of Protestants, particularly in Dublin, were middle class and, while they felt at a disadvantage in access to public services, they still had privileged access right into the 1960s in jobs in banking, insurance, accountancy, law firms and businesses such as Guinness and Elvery's. Besides, Dublin governments tended to be kind towards the middle classes in general.

This is not the place to analyse how and why problems were allowed to fester in Northern Ireland. The conflict in Northern Ireland has been the longest running one in western Europe since 1945 and probably in the whole century. Even though the Troubles occasionally spilled over south of the border, the conflict has made Northern Ireland a place apart rather than a variation on a theme.

Is the conflict over? Historians are notoriously bad at predicting the future. I have no doubt that the conflict will continue, but a conflict does not have to be conducted by violent means – it can continue the way it does, for example, in Belgium. Even if efforts to implement the Good Friday Agreement run into the sand the sort of peace we are experiencing now shows many signs that it will continue. In which case it will be: the conflict goes on, the war is over. The previously widening gulf becomes in all likelihood a narrowing one.

The sea changes north and south lead me to conclude that at the end of this decade, this century, this millennium, Northern Ireland is more like a variation on a theme than a place apart. Before I develop that thought, let me turn aside to consider the economies.

At the start of the twentieth century Belfast was one of the great industrial and trading centres of the western world. It was the third port in importance, after London and Liverpool, in the United Kingdom, then the greatest trading state on earth. Of course Belfast isn't Ulster, and the economy of Fermanagh had more in common with that of Sligo than that of the Lagan valley. But the industrialised north-east was very different from the rest of the island and from Dublin which had languished since 1815 onwards.

The North-east's economic problems began in the winter of 1920 when a severe slump developed into a protracted depression. That relative decline had more to do with seismic changes in world trading conditions than with intercommunal strife, and Belfast had much in common with British industrial ports such as Newcastle-on-Tyne, Glasgow and Liverpool. The Second World War demonstrated that the north-east still had plenty of industrial sinew and the arrival of multinational firms in the 1960s seemed to show that a new era of industrial growth was beginning. But now Northern Ireland is the least industrialised region of the United Kingdom and one of the most dependent economies in the world, relying dangerously on a massive annual subvention from the Exchequer and on public employment.

Meanwhile the Celtic Tiger roars. The island's economic experience recently has been a bit like that of Belgium: previously the French-speaking southern areas were the advanced, industrialised ones, and the Flemish-speaking regions were rural and backward. Now the Flemish north is flourishing with high-tech industry on green-field sites and the south is plagued by closing mines and steelworks. Even if we conclude that the Irish Republic's economy is dangerously overheated, the contrast in the economies north and south will remain for some time. It's too simple to say: 'Oh but the south got all that European money, and that's ending now' – that financial aid has been less per head of the population than that provided in Northern Ireland by the annual subvention from Westminster.

Really what I am saying is that the contrast between north and south is lessening and this has as much to do with changes in the Republic as with changes in Northern Ireland under direct rule. A short list of those changes would include: secularisation; economic advance and urbanisation; progress in public education; and a new willingness to acknowledge that there are elements, other than Gaelic, that have helped to shape modern Ireland (ending selective amnesia by, for example, celebrating Viking achievement in Dublin, going to Norman-revival medieval banquets, and recognising the contribution southerners made to the Allied cause in the First World War).

Other factors reducing difference include: the embracing of transnational culture (demonstrated by the MTV awards ceremony); the lowering of trade barriers within the European Union; British and Northern Ireland television programmes picked up in almost all areas of the island; fair employment and equality legislation, north and south; willingness to face up to clientelism and gross corruption; further British investment in the Republic (Tesco being a highly-visible example); investment by individual and corporate southerners in Northern Ireland (Centra, Laganside, for example); and the creeping realisation that Ireland is part of the British Isles.

For Northern Ireland much depends on what Britain will do as a member of the EU: will it go along with the other member states by, for example, adopting the Euro and going down the road towards greater integration, or will the UK decide itself to become a state apart? I have no doubt that the first alternative offers the best hope of easing Northern Ireland's internal problems. After all the EU has adopted and is implementing with a will its policy of Europe of the regions. This has the merit of facing reality: that Northern Ireland is a region of the UK and at the same time is geographically and culturally a region of Ireland, just as the whole island of Ireland is geographically and culturally a region of what used to be called the British Isles.

Northern Ireland is a region with a very special problem of ethnic division – I have no qualms about using the word 'ethnic'. Even with a copper-fastened agreement, that division is not going to be spirited away in the early years of the new millennium. The workplace is more mixed than it has ever been, thanks to some of the most draconian fair employment legislation in the world. But people here more than ever are living apart: 90% of people live in electoral districts which are 90% of one religious affiliation. Recent studies make it plain that the great majority of northern Protestants do not think of themselves as Irish at all. The majority living on the island of Ireland must live with the certainty that they are not, in the foreseeable future, going to be able to persuade these 900,000 Protestants to change the perception of their identity.

From a European – indeed a world – perspective, Northern Ireland is a variation on a theme. Until the end of the 1980s there was a widespread tendency to regard the Northern Ireland 'problem' as being a curious and unique historical survival. The coming down of the Berlin Wall ten years ago and the disintegration of the Soviet bloc indicated otherwise. There the new-found freedom allowed long-dormant ethnic rivalries to gush to the surface.

As Armenians and Azeris, and Serbs, Croats, Bosnian Muslims and ethnic Albanians slaughtered each other, it was plain they were impelled by atavistic urges remarkably parallel to those fuelling the violence in Northern Ireland. Recent Balkan horrors remind us that our protracted Troubles could have been much worse: when batteries of television cameras were turned on the confrontation at Drumcree in July 1996, bodies were being exhumed at Bukovar and Srebrenica showing that in just a few days in July the previous year at least as many were killed in those two places as in all of Northern Ireland over a quarter of a century.

Because of the very length of our conflict, we have been coming up with some very sophisticated ways of containing it. In particular, we have been devising strategies for managing and appreciating our cultural diversity and we must now have much that has been well-tested, certain to be of value to other members of the European Union.

Jonathon Bardon is the author of the monumental A History of Ulster published in 1992, and of numerous other books on historical aspects of Northern Ireland and of his native Dublin. He has worked extensively in educational and documentary television, and as a teacher of history has been involved in curriculum development.

Some navigational notes: