The signing of the European Constitution in Rome today has brought a mixed reaction in Ireland, where it will be put before the people in a referendum within the next two years.
European Union leaders signed the bloc's first constitution in a ceremony marred by a left-right dispute over the makeup of the cabinet-like European Commission.
Mr Ahern, who was heavily involved in negotiating the final details of the constitution earlier this year, told the signing ceremony today member states must ratify the historic accord quickly.
"It is of fundamental importance that all twenty-five Member States now ratify the European Constitution convincingly and on time. The process of ratification will not be easy, but, with energy and determination, it can and will be successful," he told EU leaders.
The leaders agreed the controversial charter in June after months of negotiation. It provides for a long-term president of the European Council of national leaders, a first foreign minister, more powers for parliament and eventually a smaller Commission -- but limits the democratic input of European citizens and smaller nations.
All member states must first ratify the treaty -- an unlikely prospect with at least eight, including eurosceptical Britain, planning to hold referendums over the next two years.
The leaders of France, Germany and Italy said they would start ratification procedures immediately.
The rejection of the European constitution by Irish voters in a referendum would "be an enormous setback for the country", Mr Ahern warned, at the publication yesterday of information booklets on the constitution.
People, he believed, "want to be part of the economic, social, cultural dynamic that is Europe", he said, accompanied by the Minister for Foreign Affairs, Dermot Ahern.
"To reject that and to reject the constitution would be an enormous setback in any country," he claimed, declaring that every "single Irish key interest" was protected.
Faced with previous treaty referendums, such as Maastricht, voters could identify issues they did not like "and say, 'I don't like this, or that, but I like the other,'" he said.
"When you put it into the constitution you are opposing the whole project. I do not hear people saying that," the Taoiseach said.
Irish voters will face two referendums on the Constitution, with northerners being asked to vote on British support for the charter. It was not immediately clear if the two referendums would be held in tandem.
Hardline unionists have already declared their opposition, calling today a day of shame for British Prime Minister Tony Blair.
The DUP's Jim Allister said it represented a further surrender to Brussels on British sovereignty.
With the British government focussing on a referendum on the constitution by spring 2006, he declared there would be "a day of reckoning".
"I look forward to the coming referendum when the actions of the Blair government will be called to account."