The people of Northern Ireland may well vote for their own assembly today, but the consequences will be enormous for everyone in these islands. Brendan O'Leary explains just how radical the agreement really is.

Predictions are always tricky but I am confident that the British-Irish Agreement will be endorsed in the two referendums held today. The agreement requires majority support in both the North and the South of Ireland to come into force, but its survival probably requires a 64 per cent "Yes" vote in the North. The agreement will be backed by almost all nationalists in the North, who make up 40 per cent of voters, but to work its institutions need to be actively supported by 40 per cent of non-nationalists, in other words, 24 per cent of the other voters.

The problem is that if the agreement fails to win enough backing from unionists, hard-liners in Ian Paisley's Democratic Unionist Party and dissidents in David Trimble's Ulster Unionist Party will have enough strength in the assembly elections, due to take place four weeks from today, to prevent the assembly completing its work on North-South institutions. That would kill the agreement. So a figure below 64 per cent in the North will spell serious difficulties ahead. It is possible too to predict what the likely vote shares will be in the assembly elections. According to my calculations, unionists will take 56 of the 108 seats; the nationalists will take 44. Breaking it down by party gives the UUP 29 seats, the DUP 22, the UKUP three, the Progressive Unionist Party two, the Alliance eight, the SDLP 25 and Sinn Fein 19.

If these projections are close to the mark there will be enough support for the agreement's institutions to operate. The agreement will establish an assembly, in which all members must register as unionist, nationalist or other. It will have a power-sharing executive, presided over by a first and deputy first minister who have different titles but identical powers. One unionist and one nationalist will be elected by cross-community procedures that require the candidates to be acceptable to at least 40 per cent of the other bloc's members. So even if Gerry Adams leads Sinn Fein into surpassing John Hume's SDLP in seats won, unionists will be able to block Adams's nomination as deputy first minister.

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Other ministerial posts will be allocated to parties in proportion to their strength in the assembly. Ministers will take a pledge of office requiring them to follow exclusively peaceful and democratic politics, to participate in preparing a programme of government and to follow the assembly's decisions. The duties of office include a requirement to serve all the people equally and to prevent discrimination.

The assembly is to be elected by proportional representation, using the single transferable vote, that will enable voters to transfer their lower order preferences to pro-agreement candidates and parties. The executive is to be proportional, but so too are other institutions. The agreement envisages a representative police service, a non-discriminatory civil service and vigorous fair employment throughout the public and private sectors.

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Most importantly, the agreement promises the equalisation of both national communities, as British and Irish. The European Convention (which is weak on the protection of collective rights) will be supplemented to give Northern Ireland a tailor-made Bill of Rights, to protect both national groupings as well as individuals.

The last internal dimension of the settlement is the protection of minorities. The assembly has complex cross-community devices that protect nationalists from unionist dominance. Indeed they do so in such a comprehensive manner that they may conceivably be used by hard-line unionist opponents of the agreement to wreck its initiation. In the courts, disaffected nationalists and unionists will have the means to redress breaches of their rights.

At a higher level, the agreement creates two confederal relationships. The first is all-Ireland. The North-South Ministerial Council will be established after the assembly has come into being. The assembly and the council are mutually interdependent. If the assembly does not create the council it will in effect destroy itself. The North-South Ministerial Council is the means by which nationalists hope to persuade unionists of the attractions of Irish unification; and it will link northern nationalists to their preferred nation-state. The Irish government will change its constitution to ensure that the council may exercise island-wide jurisdiction in those activities in which unionists are willing to co-operate.

The second confederal relationship affects all the islands of Britain and Ireland. In the British-Irish Council, the two sovereign governments, all the devolved governments of the UK and all the neighbouring insular dependent territories of the UK may meet, agree to delegate functions and agree common policies. This proposal meets unionists' concerns for reciprocity and provides a mechanism through which they may in future be linked to the UK, even if Northern Ireland becomes part of the Republic.

For the UK the agreement spells the end of unitary unionism, already dented by the formation of a Scottish Parliament and Welsh Assembly. The UK acknowledges that Northern Ireland has the right to secede into the Republic, on the basis of a local referendum, and recognises, in a treaty, the authority of Irish national self-determination. In international law, the UK's relationship to Northern Ireland will therefore be federal: Westminster cannot exercise power in any manner in Northern Ireland that is inconsistent with the agreement.

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The agreement also opens federalist avenues in the Republic, hitherto one of the most centralised states in Europe. The North-South Ministerial Council is seen by nationalists as a potential federalist first step. But it is far from the aggressive assimilation envisaged by hardline republicans. The Republic is bound by the agreement to unification by consent.

But the subtlest part of the agreement is its promise to entrench the identical protection of rights, collective and individual, on both sides of the present border. In effect it promises protection to Northern nationalists now on the same terms that will be given to Ulster unionists should the latter ever become a minority in a unified Ireland.

So, the agreement is immensely subtle, but why was it agreed? Nationalists have endorsed it because it gives them equality now with the possibility of Irish unification later. They get to co-govern the North rather than being simply governed by unionists or the British government. Moreover, they get this share of power with promises of further reforms. Republicans in Sinn Fein and the IRA can trade a long war that they could not win - and could not lose - for a long march through institutions in which they can reasonably claim that only their means have changed, not their end: the termination of partition.

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But why did any unionists and loyalists make this bargain? The answer does not lie in the charms of Tony Blair, the diplomacy of George Mitchell or in the negotiating process. Unionists wish to safeguard the Union. Those who signed the agreement calculate that they can only reconcile nationalists to the Union, especially in the light of shifts in the balance of demographic power, by being generous now. Unionists get a share in self-government, avoid the prospect of a British government making further deals over their heads with the Irish state, and have some prospect of persuading northern nationalists that the new reconstructed Union is a secure home for them.

The beauty of the agreement is that both nationalists and unionists have sound reasons for believing that they are right about the long term. Because they cannot be certain they are right they are willing to make this elaborate settlement now.

But even if the agreement has a good beginning will it die once it becomes apparent who is right about the long term? That possibility cannot be excluded, but that is why the agreement repays inspection. There are incentives for each bloc to accommodate the other precisely in order to make its vision of the future more likely. The confederalising and federalising possibilities ensure that both national communities will remain linked, come what may, to their preferred nation-states.

That is not to say that presiding over the twilight of the second Protestant ascendancy in Irish history and managing the rerustication of militant republicanism will be easy tasks, but the agreement has already digested many impossibilities before its first breakfast.

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Brendan O'Leary, professor of political science at the London School of Economics, was born in the Republic of Ireland and grew up in Northern Ireland. He is a former adviser to Marjorie Mowlam, secretary of state for Northern Ireland.