It was 4.23 a.m. when they finally did it. A last reassurance from his partners on a technical aspect of voting, brokered by the Belgian Prime Minister, Mr Guy Verhofstadt, allowed Belgium to live with the indignity of having fewer votes than the Dutch. Later more than one paid tribute to the "statesmanlike" Mr Verhofstadt and his willingness to compromise.
Tempers had frayed as the meeting dragged on and a succession of glitches were removed from the "house of cards", as France's European Affairs Minister, Mr Pierre Moscovici, described the delicate balancing act of re weighting member states' votes.
Earlier an exasperated Italian Prime Minister, Mr Giuliano Amato, had pointedly brandished his watch with the stage whisper: "Soon time for the Swedish presidency". The Austrian Chancellor, Mr Wolfgang Schussel, said later: "Success was in doubt to the very end."
The Portuguese had reacted particularly badly to what they saw as French bullying over voting weights. "This is a coup d'etat by the big member states," their Prime Minister, Mr Antonio Guterres, said about an early draft compromise.
And then there were late surprises too. Out of nowhere, though parentage has been claimed by both the German Chancellor, Mr Gerhard Schroder, and the Luxembourg Prime Minister, Mr Jean Claude Juncker, came the idea of electing the future presidents of the Commission, and separately his team, by qualified majority.
All remember too well the humiliation in 1994 of the Belgian candidate, Mr Jean Luc Dehaene, by a British veto. Carried unanimously.
The total package of treaty reform, supposed to pave the way for enlargement, will scarcely impress. But leaders were quick to insist it was enough for the enlargement process to carry on.
More than 40 policy areas have been transferred to majority voting, the Commission membership will be capped at 27, and the concerns of larger states that they may be swamped by an influx of smaller ones have been allayed with a radical reweighting of votes.
However, it has been made more difficult to reach a qualified majority, and some member states are bitter that they cannot contemplate tax harmonisation. The Taoiseach, Mr Ahern, and the British Prime Minister, Mr Blair, held the line. For now.
But it will not go away as an issue, as the representative of the Parliament, Mr Elmar Brok, pointed out, in such crucial areas as e-commerce.
The Commission President, Mr Romano Prodi, who had been making clear his frustration earlier, said that he would have preferred more majority voting, but "now enlargement can really take place effectively". The onus was on the accession countries to prepare now.
Senior Commission sources say the deal was probably the best that could have been attained in the circumstances.
Mr Schroder said that he had started with one strategic goal - "that Europe should be united. This is a deal that will make enlargement possible."
He had wished for more majority voting, he said. "But if you look at what we achieved you will know that we could not have dreamed of this a few years ago."
The French who had made all this possible - or so they said - were calling it a historic occasion. But their stock has suffered from what many saw as both bullying and an absence of strategy.
Experienced diplomats from many states simply shook their heads in bewilderment. Relations between small and large states were strained and the former were driven together in an otherwise unusual informal alliance, which does not bode well for the future.
Yet for many Brussels observers it was nearly not historic at all.
Until the last hours there were still huge differences over two major veto issues which the Commission regarded as vital to its coherence and efficiency - drawing trade negotiations in services and intellectual property under the Community wing, and setting rules and maps for structural and cohesion funds.
Although the French kept their exception for films and TV in the former, and the veto will not go in the latter until 2007, important decisions of principle were made.
Failure to make these breakthroughs could well have led to a determined campaign by MEPs to block ratification.
The enthusiastic backing for the so-called Post-Nice Agenda reflects the extent to which a convergence of some of its key elements reflect a convergence of interests between Europhiles and Eurosceptics.
Ireland will have to use its 2004 presidency not only to manage enlargement, which should be well under way, but to steer preparations of another, hugely complex Inter-Governmental Conference.
Its remit is to discuss the simplification of the treaties, the final status of the Charter of Fundamental Rights, the possible drafting of a constitution for the EU, and an exercise in defining precisely who does what in Europe.
Mr Blair was a strong supporter of the German-inspired agenda, arguing that the Union has a "fundamental question to face and that is the notion that Europe should do the things it really needs to do, but should get out of areas where it is not needed."
If the current IGC's painful labours are anything to go by, leaders would want to block out at least a month of their diaries for the task.