The Taoiseach, Mr Ahern, said after the formal announcement of the referendum result at Dublin Castle this evening that work at EU level on enlargement would begin immediately.
He said it was now time to move on and that the European Council meeting in Brussels at the end of week would be almost exclusively devoted to enlargement.
"Now that the last obstacle to the ratification of the Treaty of Nice has been removed the European Union can move to meet the challenges and opportunities that we in Europe will face together," he said.
"We can now ratify the Treaty of Nice and the truly historic enlargement of the European Union can go ahead."
He said today's vote showed Ireland was committed to the European Union and that the Irish realise "what is good for the people of Europe is good for the people of Ireland".
A huge cheer went up around the count centre as the Taoiseach said: "This decision shows most of all that we are a nation that wants to welcome the peoples of the applicant countries with open arms.
"From our own perspective this decision ensures that Ireland remains at the heart of Europe where we belong, where we can continue to make a valuable contribution and we can ensure our interests can continue to be protected."
He acknowledged that feelings were running high during the campaign and said he recognised some of the concerns. He said he was looking at ways to ensure the people would be better informed about the EU and that the Union must be closer to its citizens.
"We must now ensure that our original enthusiasm for the great European project is rebuilt.
"Last year the Government took on board the serious concerns expressed by our people about the protection of our neutrality and we were happy to propose further protection for our traditional policy by including it in our constitution."
He concluded by saying Ireland was looking forward to welcoming the applicant countries during Ireland's presidency of the European Union.