The Taoiseach and the British Prime Minister have staked their political reputations on the comprehensive peace package agreed by the parties in the Stormont talks yesterday after almost two years of negotiations culminated in a marathon final session in which the two leaders played leading roles.
Mr Ahern said: "This is a day we should treasure - a day when agreement and accommodation have replaced days of difference and division."
In a joint press conference with the Taoiseach, Mr Blair said: "I believe today courage has triumphed. I said when I arrived here that I felt the hand of history upon us. Today I hope that the burden of history can at long last start to be lifted from our shoulders."
The two leaders have made a huge investment of time and energy in the peace effort, which ended in an extraordinary final push that lasted almost 36 hours without a break.
The talks chairman, Senator George Mitchell, praised Mr Blair and Mr Ahern. "I cannot think of a comparable instance when two leaders of governments came and participated in a round-the-clock, hands-on basis for several days as they have done," he said.
The SDLP leader, Mr John Hume said: "Only once in a generation does an opportunity like this come along, an opportunity to resolve our deep and tragic conflict."
President Clinton intervened in the talks at the last minute to help allay unionist fears over the decommissioning of paramilitary weapons. There were signs of unionist nerves as the talks drew to a close, and for a short period yesterday afternoon it seemed the deal might not go through after all.
Sources said the UUP leader, Mr David Trimble, took a determined approach at a meeting of his party delegation before agreeing the deal. "I am doing it," he said, and left the room.
His fellow MP, Mr Jeffrey Donaldson, was said to be particularly anxious over decommissioning, and there was intense speculation about internal disagreement when he left before the end of yesterday's session. Mr Trimble said Mr Donaldson had "got his own commitments". Another UUP source said Mr Donaldson was going on holiday.
Mr Trimble said the last-minute delay was "justified" by his party's "very serious concern" about the proposed assembly. But he was much relieved by assurances given to the unionists by Mr Blair.
In a letter to Mr Trimble, the Prime Minister said that if provisions on decommissioning were shown to be ineffective, the British government would support changes to ensure that people connected with paramilitary organisations that still held their weapons would not hold political office.
"Furthermore, I confirm that in our view the effect of the decommissioning section of the agreement, with decommissioning schemes coming into effect in June, is that the process of decommissioning should begin straight away," Mr Blair said.
Mr Trimble declared: "I have risen from this table with the Union stronger than when I sat down." He said the UUP had "saved the future of the RUC" in the negotiations. Echoing sentiments expressed from the other side of the political divide by republican participants in the talks, Mr Trimble said: "It's not perfect, it's the best we can get at the moment."
He pointed out in a UTV interview that the UUP was getting the Republic's territorial claim removed, and claimed there were no significant changes in the UK's constitutional legislation on Northern Ireland.
The agreement exceeded Senator Mitchell's midnight deadline by almost 18 hours. Tension eased at 5.36 p.m. when Mr Mitchell declared: "I am pleased to announce that the two governments and the political parties in Northern Ireland have reached agreement." Politicians, some of them sworn enemies, applauded him and his team.
Mr Trimble said last night: "We know that the consent principle has been accepted by all nationalists, but with one conspicuous exception, and will be accepted and enshrined in the fundamental law of the Irish Republic."
Asked when he would be speaking face to face with Mr Adams, he told a press conference he would be prepared to deal with Sinn Fein as democratic politicians.
Sinn Fein was reserving its position on the document and the party president, Mr Gerry Adams, said that while the paper contained elements which were positive, "there are others yet to be resolved".
Sinn Fein is expected to hold a meeting of its ardchomhairle next week to hear a report on the talks from its negotiators. The executive will assess the document and may recommend it for discussion at the party's ardfheis in Dublin next weekend or it may hold a special ardfheis on the matter.
The former Taoiseach, Mr Albert Reynolds, who with Mr John Major, played a significant early role in the peace process, said: "It is a new beginning. There is a lot of hard work ahead. The feeling of history in the making is there. It's nice to be a political leader who sets out on a mission and gets there before he is buried."
The Minister for Foreign Affairs, Mr Andrews, said it was truly a day when history had been made. "The success of these negotiations will, I believe, come to be seen as a turning point in the affairs of this island."
The Northern Ireland Women's Coalition said: "We owe it to all those who have suffered and died over the past three decades to grasp this opportunity to build a society that will stand as a living testimony to the victims of the Troubles."
The RUC Chief Constable, Mr Ronnie Flanagan, said the force would be "vigilant in the days and weeks ahead".