The Taoiseach Mr Ahern this morning bid a "Cead Mile Failte" to the ten new EU member states and said today marked a new beginning for all Europeans.
"Over the past years, you have been knocking on the door of Europe's biggest family. Today, we open it and in the great Irish tradition, bid you a "céad míle fáilte" - a hundred, thousand welcomes," he said at a meeting betwen himself, the European Parliament President Mr Pat Cox and the European Commission President MR Romano Prodi in Dublin Castle.
Speaking at the beginning of the Day of Welcomes in Dublin, Mr Ahern spoke of the need to fight the "scourge of terrorism" and said the first major task for the newly expanded Union was to agree a new constitutional treaty by the end of June.
While acknowledging large parts of the treaty had already been agreed, he expressed his hope that a way forward on the "more difficult and sensitive issues" could be found before the end of the Irish presidency of the EU on June 30th.
"The Constitutional Treaty will help to ensure that the new needs and circumstances of the European Union are reflected in its basic law," he said.
"It will improve the foundations for democratic discourse in the European Union. It will build on the success of the European Union and it will make the work of the Union more accessible and effective."
Sitting alongside Mr Cox and Mr Prodi, Mr Ahern said many other challenges faced the new EU.
"We will have to continue our work to protect our citizens from the scourge of terrorism," he said.
"We will have to work harder to create jobs and growth. And many outside the European Union look to us to assist them in building the peace and prosperity we enjoy. There is indeed much work to be done."
Mr Ahern issued a warm welcome to all the new member-states and said today was a time for celebrations.
"To the people of Europe who are joining us today in the European Union I extend the hand of friendship," he said.
Bertie Ahern
"Our Union will change with twenty-five members. That is inevitable. But that change will be for the better. The Union has been strengthened and enhanced by the contributions of each of its current members. Ten more members will enhance and strengthen the Union that much more," he said
"It was your democratic choice and your own efforts that made this day happen.
"Today marks the triumph of your determination and perseverance over the legacy of history.
"For Europe, today marks the closure of one chapter and the opening of another new and exciting chapter in its long history."