The new constitution, agreed upon six months behind schedule in Brussels, was hailed by EU leaders as a "win-win" success for the expanding bloc.
"This is a major, major success for the presidency," said Brigid Laffan, professor of European Politics at the Dublin European Institute, an independent thinktank.
Ireland, the holders of the EU's six-month rotating presidency, brokered a deal on the treaty late on Friday after exhaustive diplomacy. If ratified, the constitution will serve as the 25-nation bloc's guiding document for the foreseeable future.
European Parliament's president, Pat Cox - another Irishman who might still be in with a chance to become European Commission president - said: "This represents a victory for quiet, patient and inclusive diplomacy and the politics of the Irish presidency."
In shepherding the EU to a deal on its first constitution, Irish Prime Minister Bertie Ahern may have pulled off an audacious coup which will be remembered as a historic step forward for the continent.
It will go some way to restoring his battered standing at home following last week's poor European and local election results, and may even boost his chances of emerging as the next head of the European Commission, whether he likes it or not.
Irish PM Bertie Ahern (R) helped
pave the way for a consensus
Who heads the commission has been a contentious issue and has already exposed lingering divisions between member states over the US-led war in Iraq and deep-seated differences in their vision of European integration.
The last attempt to agree on a constitution in December
collapsed spectacularly after a battle over the power-charged issue of voting rights in the EU, which took in 10 new mostly ex-communist member states on 1 May.
Six months ago Poland and Spain notably held out against
proposed changes that would have seen the generous voting powers they currently enjoy eroded.
Devil in details
Although by day's end, EU leaders were toasting one another over the constitution, the final text still includes provisions which will likely spark fiery debate and discussion.
One provision for so-called double-majority voting will be at the heart of EU decision-making once the new constitution is ratified in all 25 states.
Under the new arrangements, new EU laws will need the approval of 15 of the member states, representing at least 65 percent of the entire population of the bloc.
Crucially, they also set out strict rules for a "blocking minority" - effectively ensuring that EU heavyweights Britain, France and Germany cannot themselves throw out a piece of legislation.
Also under the new constitution the EU's executive commission will be streamlined from 2014, while the European Parliament's size will be capped at 750 lawmakers and small EU states will be guaranteed a minimum of six seats.
Clashes among the heavyweights had cast a long shadow over the talks.
Britain fired back at an ill-disguised jab from Chirac that London risked blocking the EU by its insistence on sticking to its constitutional red lines - such as no loss of national veto over taxation.