The collapse of last month's summit in Brussels has left the Irish presidency to decide how to proceed with negotiations towards a constitutional treaty.
The summit asked the Taoiseach to report to a summit in March on the likelihood of finding agreement, and Mr Ahern will spend the first weeks of the presidency consulting with other leaders about the possible shape of a deal.
Some EU leaders suggested after December's summit that it would be best to wait for up to a year before resuming negotiations. This is a view shared by the former chairman of the Convention on the Future of Europe, Mr Valery Giscard d'Estaing.
There are fears, however, that a long delay could allow compromises to unravel amid popular doubts about the wisdom or necessity of the entire constitutional project.
The 25 present and future EU member-states agree on almost everything in the draft constitution, although a number of issues of concern to particular countries, including Ireland's demand to retain the national veto on all tax issues, remain to be agreed.
The most important dispute concerns how countries should vote in the Council of Ministers, with France and Germany leading calls for a system that reflects population size, while Spain and Poland insist that the system of weighted votes agreed at Nice should be retained.
Under Nice, Spain and Poland each have almost as many votes as Germany, despite the fact that their populations are less than half the size of Germany's. Berlin and Paris want decisions to be made in future by a double majority - defined as a majority of member-states representing at least 60 per cent of the EU's population.
The Brussels summit failed because these four countries were unable to reach a compromise, and the prospects of an early resumption of negotiations will depend on their willingness to co-operate.
Germany's Chancellor, Mr Gerhard Schröder, has called for an early conclusion of negotiations, and Berlin has a clear interest in reaching a deal based on the draft constitutional treaty as soon as possible.
Germany stands to gain more than any other member-state from a voting system based on population size. The imminent start of talks on the EU's seven-year budget - to which Berlin is the biggest contributor - strengthens Mr Schröder's negotiating hand.
Spain's Prime Minister, Mr Jose Maria Aznar, has also called for an early resumption of treaty negotiations, despite the fact that he will step down in advance of Spain's general election in March.
As a major recipient of EU funds, Spain wishes to avoid any negative repercussions a treaty deadlock could have on the budget negotiations. Mr Aznar is aware that a good deal for Spain on the constitutional treaty will be no easier to achieve later than sooner.
Despite its tough rhetoric during the treaty negotiations, Poland has indicated a willingness to compromise on its demand for the retention of the Nice voting system, and Polish officials believe they have been blamed unfairly for last month's failure to agree.
As a new member-state, Poland wishes to assert itself as an important player in the EU but has little interest in establishing an early reputation as a stubborn negotiator lacking in the European spirit.
The biggest doubts surround France's willingness to make a deal, and many EU diplomats believe that French intransigence was to blame for last month's impasse.
France believes it has already made too many compromises in the treaty negotiations, and is unwilling to give ground on the voting system. Besides, the French President, Mr Jacques Chirac, has made clear that he sees the prospect of a two-speed Europe as a chance for like-minded countries to accelerate the process of European integration after enlargement.
The Taoiseach, an experienced negotiator who enjoys good relations with other EU leaders, is as well-placed as anyone to find a compromise if the parties are willing to make a deal.
Ireland is neutral on the voting question and Mr Ahern has shown understanding for both positions on the issue.
A successful conclusion of negotiations during the next six months would represent a major achievement for Mr Ahern and would ensure the success of the Irish presidency. Failure to agree during the presidency would not be seen as an Irish failure but it could have important consequences for Europe's future.
The longer a deal is delayed, the more questions will be raised about the constitutional treaty.
The pressure will become greater for a core group of EU member-states to press ahead with closer union, leaving the rest to catch up when they can.