The Taoiseach has said changes in EU decision-making procedures are now essential, as he begins a tour this week of European capitals for final discussions on the proposed EU Constitutional Treaty.
On Saturday, Mr Ahern had separate meetings with the prime ministers of Spain and Slovenia while he had discussions with Mr Tony Blair in Dublin yesterday.
At a press conference in Dublin castle on Saturday with the European Commission president, Mr Romano Prodi, and Parliament president, Mr Pat Cox, Mr Ahern said the "Day of Welcomes" ceremony marked a historic European moment.
"Today is the day after 15 years of huge effort, changing the course of history, ending the divisions of Europe, it's a day the leaders of Europe should be together to acknowledge what has been achieved and to pledge ourselves to work together for the future."
He said that while the leaders were not having a formal meeting at the weekend, they would have discussions on the constitution. "I do start this week the tour to meet all my colleagues individually to discuss the European Council."
He said changes in EU decision-making structures were essential. "We have to have an effective system that can make decisions, that can process the enormous challenging issues that are always in front of us."
He said there were difficult decisions to be reached on the constitution, on the EU's future budget and on the prospect of further enlargement to include Bulgaria, Romania and Turkey.
"What we need to do in a larger Europe is have a system that allowed us to do that." There would be enormous difficulties if the EU tried to continue with 25 members under existing rules.
Mr Prodi rejected a suggestion that enlargement would make it even harder to reach unanimous decisions on contentious issues such as the war on Iraq.
He noted that the divisions on Iraq were among the old 15 member states rather than the enlarged 25 states. "We could not be more split than we are now. The problem is not the number of countries, the problem is rules. If we have a new constitution progressing the rules in order to take a decision, we shall be able to take a decision. If we go along with unanimity we will be unable to take a decision even if we have just three countries and not 25."
The Taoiseach also said he did not believe there would be a reference to God in the new constitution, as has been demanded by several member states. There would be a reference to religious values, he said, but "I do not think we will get a reference to God. I think the wording that is there now will probably stay the same. There might be some change, but I do not think it is going to change substantially. I know some countries are continuing to keep the issue open but it is not something that has been raised much with me in recent weeks".
Asked whether restrictions on the right to work and welfare entitlements imposed by existing member-states on citizens of the new states had tarnished the message of welcome, Mr Ahern said that Ireland was the only country without any restriction on the right of people from the new member-states to work.
He refused to be drawn on who would be the next president of the European Commission, or on how highly he rated the current British EU commissioner, Mr Chris Patten, who the Financial Times reported on Saturday, was now seen as a candidate for the post.
"There are a number of very prominent and very outstanding people's names being floated, and some of them are being seriously discussed," he said.
"Over the next five or six weeks it will be my task to talk to all of my colleagues. I do believe we will get an extremely good president of the Commission from the names that are being mentioned."
British Prime Minister Mr Tony Blair warmly congratulated Mr Ahern yesterday following Saturday's "quite magnificent" celebrations in the Phoenix Park.
Speaking after meeting Mr Ahern in Dublin, he said:"Can I congratulate the Taoiseach, the Irish Government, the Irish people for the quite magnificent celebrations yesterday for the European Union enlargement."
"I have been going to EU events for seven years and I honestly cannot remember a display that was more impressive, or more moving and that entirely fitted the occasion."