Mr Ahern has his work cut out if he wants to resolve EU differences over a new treaty. Denis Staunton examines the positions.
As the Taoiseach begins intensive discussions with EU leaders in an effort to revive talks on Europe's constitutional treaty, his focus will be on the most contentious issue - the introduction of a new voting system in the Council of Ministers.
Last month's summit in Brussels collapsed after the Italian Presidency failed to find a compromise between France and Germany, which want a system based on population size, and Spain and Poland, which want to retain the system of weighted votes agreed at Nice.
So, who wants what and can a deal be done?
Germany: As the EU's most populous member-state, Germany stands to gain most from the new voting system proposed in the draft constitutional treaty. This would allow any measure to be approved by the Council of Ministers if it had the backing of a majority of member-states representing at least 60 per cent of the EU's population.
The chancellor, Mr Gerhard Schröder, said this week that he wants a deal on the constitutional treaty before the end of 2004, but he ruled out a compromise on the principle of linking votes to population size. Before last month's summit, however, German officials hinted that Berlin could compromise on the population threshold, allowing it to be raised from 60 per cent.
Mr Schröder's enthusiasm for an early deal could be enhanced by the prospect of a decision by EU leaders in December on whether to begin negotiations with Turkey on EU membership. If talks on the constitutional treaty continue into the Dutch Presidency, which follows Ireland's, the treaty could be agreed at the same summit that determines Turkey's readiness to start accession negotiations.
Germany's opposition Christian Democrats have made clear that they will oppose the start of talks with Turkey and Mr Schröder will be unwilling to allow the issues of Turkey and the constitutional treaty to be linked in Germany's public debate. Of the four countries at the heart of the disagreement over voting weights, Germany is probably the most prepared to reach an early agreement.
Spain: Under the Nice Treaty, Spain has 27 votes in the Council of Ministers, compared to 29 held by each of the EU's four biggest states. This is despite the fact that Spain's population, at 40 million, is less than half that of Germany.
Throughout the 18 months of the Convention on the Future of Europe, Spain made clear it would not accept any departure from the voting arrangements agreed at Nice. In keeping with its reputation as the most stubborn negotiator in the EU, Madrid held firm to its position throughout the talks that led to last month's summit.
Talks on the constitutional treaty are complicated by the fact that Spain will hold a general election in March, around the same time as the Taoiseach is due to report to an EU summit on the prospects of finding a deal. The prime minister, Mr José Maria Aznar, will step down at the election but his conservative Popular Party is expected to be returned to government.
Despite Spain's stubbornness, its negotiators claimed in Brussels last month that they were willing to make a deal before the talks collapsed. They complained that no agreement was possible because Italy's prime minister, Mr Silvio Berlusconi, failed to table a compromise proposal.
Spain is a major beneficiary of EU regional funds and its government may be reluctant to allow the treaty talks to become entangled with negotiations on the EU's next seven-year budget plan, which will begin later this month. A compromise acceptable to Spain would probably involve delaying the introduction of the new voting system to 2013 and could see the restoration of its right, with the EU's five other big states, to nominate two members of the European Commission.
Poland: Poland's prime minister, Mr Leszek Miller, had to be wheeled into last month's summit following a helicopter accident, but his injuries did nothing to diminish his determination to hold Poland's line on the voting issue. With a population similar to Spain's, Poland carries disproportionate weight under the Nice voting system and Mr Miller's resistance to change in Brussels was popular at home.
The government's political weakness makes compromise difficult for Mr Miller, not least because many Poles fear that they will not be treated with the respect due to a great European nation after they join the EU. Some Polish officials, particularly in the foreign ministry, are concerned, however, that Warsaw's tough stance has allowed Poland to be made a scapegoat for the summit's collapse.
They are unhappy with Poland's growing reputation in Brussels as a country that stubbornly puts its national interests before those of Europe as a whole. Polish diplomats are also eager to balance their commitment to a strong transatlantic relationship with an active engagement in EU affairs.
France and Germany have dropped broad hints that, if Poland blocks a deal on the constitutional treaty, it will be punished during the budget negotiations. Polish officials claim, however, that they did not form the biggest obstacle to finding a deal in Brussels and hint that they could fall into line with a compromise acceptable to Spain.
France: France's president, Mr Jacques Chirac, has told the Taoiseach he will support any effort to break the deadlock on the constitutional treaty. Many Brussels insiders, however, believe last month's summit failed because France would not compromise.
French officials claim Paris had already made more compromises on the treaty than it wished to in advance of the Brussels summit and that it was better to have no constitutional treaty than to have a bad one. Mr Chirac's renewed enthusiasm for a "two-speed Europe" has provoked suspicions that he has lost interest in the constitutional treaty and wishes to pursue closer integration through a core group of like-minded countries.
Since the summit's collapse, senior French officials have examined the possibility of using the EU's enhanced co-operation mechanism to co-operate with other countries on various policy issues. Sources suggest, however, that the outcome of their analysis has been disappointing so far.
Of the four leaders at the centre of the dispute, Mr Chirac is likely to be the least enthusiastic about an early resumption of the constitutional talks.