On New Year's Day, 1801, precisely 200 years ago, the Act of Union between Great Britain and Ireland was implemented. The Belfast Newsletter, the oldest surviving newspaper title in these islands, reported the event, the hoisting of the Union flag and the salute by the Royal Artillery. But the Newsletter also commented on the political significance. It acknowledged that some of the people of Ireland had been "inimical" to the measure but it was now the "interest" as well as the "duty" of everyone to bury all "all religious animosities." The new kingdom should be seen as made up of "one people" with common objectives: "tranquillity, prosperity and security."

"One people" is the constantly reiterated theme of those who advocated the Union. The dreadful murderous horrors of 1798 were to be forgotten; Irish and English, Catholic and Protestant were now to consider themselves one people. A shared and common path of progressive "liberal" (another much-used word) development was surely preferable to Anglo-Irish conflict and sectarian rage working only to the benefit of the common enemy, France.

For one key group, it worked. In his thoughtful survey of the first century of the working of the Union, W. O'Connor Morris saw only one unambiguous triumph.

One important element in Irish society, the Ulster Presbyterians, whose loyalty to the Crown was much in doubt in 1798, had been won over. In truth, this development seems to have taken place quickly. The execution of key Presbyterian United Irishmen leaders, the degeneration of the French Revolution into a military dictatorship and, above all, the renewed fear of Catholic sectarianism after 1798 had all combined to change attitudes.

As early as 1803, in the aftermath of Robert Emmet's abortive revolt, Presbyterian liberal intellectuals who had once been happy to identify with the "mere Irish" now explicitly demanded the blessings of a reformed "British" constitution: a judgment confirmed by the economic success of Belfast in the 19th century.

But, Ulster Presbyterians apart, throughout most of the 19th century most Irish people took a more jaundiced view of the Union. They tended to agree with Henry Grattan's 1799 critique of the project; it was not, said Grattan, the creation of one people, the identification of two nations.

Rather, he said, it was the merger of the parliament of one nation with that of another; the complete and utter absorption of the Irish body politic in the suffocating embrace of a larger, unsympathetic entity. Moreover, the self-destruction of the Irish parliament was achieved only by colossal bribery.

O'Neil Daunt asked: if French gold bribed the English House of Commons into a legislative union with France, how would the English people like it?

The emphasis on bribery, which was, in truth, both unusual in scale and illegal, has served to take attention away from those Irish politicians who sincerely supported the Union on substantive intellectual grounds.

For example, when William Cusack Smith's speech in favour of the Union was published, it quickly ran through six editions, creating "a powerful and extensive sensation."

The prime minister, William Pitt, immediately asked to meet the author. It was hardly a surprise; Smith had been an intellectual friend and ally of Edmund Burke until Burke's death in 1797.

Indeed, his speech reminded everyone of this point, whilst Pitt's language in support of the Union - his references to the unacceptable exclusion of Irish Catholics from political power - was seen by critics as evidence of the "uncontrolled influence" of the "Burkist system of thought."

Smith, it is clear, believed in Pitt's rhetoric, the promises of economic development and equality of esteem for both Irish and English identities. Yet within five years William Smith was to be sharply disillusioned.

In a private correspondence with another equally self-conscious Irish Burkean intellectual, J.W. Croker, Smith complained bitterly that England had not delivered on its side of the bargain. It was Burke's worst fear - the level of English interest was simply not there.

Smith asked a pertinent question; if he felt the deficiencies of the Union so strongly, how did the people who had never supported it in the first place feel? The key problem here was the failure of the British government to live up to its hints that the Union would be rapidly followed by Catholic emancipation. This did not happen and worst still, in the very early years of the Union, the government of Ireland was placed in the hands of men who still believed in "Protestantising" Ireland.

Catholic emancipation did not follow until 1829; and then only because Daniel O'Connell's agitation forced it out of the British state. Up to 1815 at least, O'Connell insisted that Pitt had deliberately cheated the Irish people on this matter. In the late 1820s, O'Connell changed his mind and decided that Pitt had been sincere.

It makes little difference: the forces of political Protestantism concentrated around George III vetoed Catholic emancipation and thus created the conditions for a mass Catholic nationalism which ultimately subverted the Union, working on the historic consciousness of Mr Gladstone who, looking back, saw his hero Peel's hostility to O'Connell as a "black page" in British history.

From the mid-1880s onwards, following the Gladstone government's conversion to Home Rule, a new type of unionism is born. There is no longer any talk of creating one people loyal to the United Kingdom in both Irelands.

The key tenet of the new unionism is intellectually much less ambitious but also much more realistic. Simply stated, it claimed that there was no argument for granting a Dublin parliament which could not also be used to support the case for a Belfast parliament.

Lloyd George replied to de Valera's cry of British coercion at a key moment in 1919: "This gentleman [de Valera] speaks about forcing authority upon people by arms. In principle it is the same whether you are forcing one and ahalf million or three million people". Lloyd George went on to be more explicit: Irish republicans were "not satisfied in getting self-determination for themselves without depriving others of the right of self-determination."

We are living in changed times. We are told that the selfish strategic interest so explicit in British policy in 1800 no longer applies. Mainstream Irish republicans no longer believe in coercion. But the Union enters its third century in Northern Ireland just as it began its life, as the British state's best guess as to how to contain Catholic and Protestant antagonisms on the island.

The crucial new element is in the Irish State's explicit support. The elaborate provisions of the Belfast Agreement - the double consent model of the whole structure - should not obscure this. Stability on the island means the survival of the Union for the foreseeable future - beyond his own lifetime, says Tony Blair.

William Pitt and his successor are very different men; but in this one respect they continue to inhabit the same world.

Paul Bew is professor of politics at Queen's University, Belfast.