Almost four million people are eligible to vote in concurrent referendums on the Belfast Agreement today, the first all-Ireland poll since 1918.
By the simple stroke of a pen, North and South, they will determine the march of Irish history towards the close of another century when they vote on the most significant political initiative since the 1921 Anglo-Irish Treaty.
With voters south of the Border also voting on the EU's Amsterdam Treaty, political sources believe that the turnout in the Republic could be the highest in the history of referendums, topping the 75.8 per cent poll on the draft constitution in 1937. The projections for the North are less certain.
People on the whole island will be asked to ratify the Belfast Agreement when polling stations open at 7 a.m. in Northern Ireland and 8 a.m. in the Republic. But the question will be posed in different ways in the two jurisdictions.
Today's vote is the culmination of two years of talks at Stormont chaired by Senator George Mitchell.
The referendum campaign has been bitter and intense and if, as expected, the agreement is approved by a majority, the subsequent elections to a new, 108member Northern assembly are likely to be the most sharply-contested in Northern Ireland's history.
The 1,175,741 voters in the North will be posed with just one question: "Do you support the Agreement reached in the multiparty talks on Northern Ireland and set out in Command Paper 3883?"
The 2,747,088 voters in the Republic will be asked to make two decisions: to ratify the Belfast Agreement on the white ballot paper and the Amsterdam Treaty on the green ballot paper.
The six constitutional amendments required to give effect to the Belfast Agreement, including the changes to Articles 2 and 3, will be posed in a catch-all question: "British-Irish Agreement: Do you approve of the proposal to amend the Constitution contained in the undermentioned Bill? Nineteenth Amendment of the Constitution Bill, 1998".
The Northern vote is being held on a single constituency basis while, in the South, it is based on the 41 Dail constituencies. The results should be known by 1 p.m. tomorrow in the North and by 6 p.m. in the Republic.
A sense of momentous opportunity in Irish affairs could be gleaned from the eve-of-poll statements by the political leaders.
Urging voters to keep their appointment with history, the Taoiseach, Mr Ahern, said: "We have talked for 30 years about this day. Up to this, we have done all the things a democratic society can do for peace - we have prayed for peace, gone on trains for peace, marched for peace and campaigned and negotiated for peace. But this is the first opportunity we have had to vote for peace, and we must grasp this chance".
The size of the Yes majority in today's poll will give an indication of the support for the stance taken in support of the agreement by the leader of the Ulster Unionist Party, Mr David Trimble.
The ideal result, from Mr Trimble's point of view, would be a Yes vote of 75 per cent or higher. This would be a serious blow to opponents of the agreement, who have claimed that a 26 per cent No vote would mean that a majority of unionists are opposed to the agreement.
A Yes vote below 60 per cent would be severely damaging to unionists supporting the agreement and would probably precipitate a leadership crisis for Mr Trimble.
A decisive victory in the referendum will allow Mr Trimble to be portrayed as the undisputed political leader of the unionist community and will constitute a major setback for his rivals inside and outside his party. anaiste, Ms Harney, said that the people of Ireland would be involved in something truly historic today. It was inconceivable that anyone who was able to would not vote.
The Labour Party leader, Mr Ruairi Quinn, urged people not to let the opportunity to create a new beginning for all to slip from their grasp. In Northern Ireland, voting begins at 7 a.m. and will continue until 10 p.m. at 1,228 polling stations. Turnout is expected to be considerably higher than in a general election, with estimates ranging from 75 to 85 per cent.
The North's chief electoral officer, Mr Pat Bradley, said that the interest expressed by the general public had been "immense". Television viewing figures are also reported to be very high.