The Taoiseach Mr Ahern this evening hailed the Yes vote in the Nice Treaty referendum as "historic", saying the Irish people had given an "emphatic yes" to European Union enlargement.
However, many who had campaigned for a No vote warned the issues raised during the campaign had not gone away.
The Green Party claimed the Government and Opposition parties who backed the Treaty had intimidated the electorate.
Mr Ahern said there had been "an excellent turnout" and Irish voters, in overturning the results of a referendum last year, had given "an emphatic Yes to enlargement".
The Fine Gael leader Mr Enda Kenny said Ireland's Yes to Nice is an "historic decision for democracy and for the future of Europe."
"Ireland's Yes proves the sophistication of the electorate," he added.
Minister for Transport, Mr Seamus Brennan, said: "The result is obviously emphatic."
Mr Brennan said: "I think people looked at the bigger picture and the bigger argument - that whatever concerns there were about the detail of this, the issue was enlargement and the economy.
Green Party MEP Ms Patricia McKenna, who campaigned for a No vote, conceded defeat and said "the people have spoken and we will have to accept their decision".
"As a democrat, unlike the Government, I would accept the result. If that's the result the people have voted they have made their decision, and their decision should be respected," she said.
Mr Justin Barret of the No to Nice coalition called it a sorry day for democracy saying: "The Irish people were put under enormous pressure. They were bullied and threatened throughout the campaign and if they have succeeded by this method then really its a very sorry day for democracy."
Mr Anthony Coughlan of the National Platform also complained about the Yes campaign and said the result showed the Government and not the people are "the masters".
"On a dark day for democracy in Ireland and in Europe, Irish voters have succumbed to threats, pressure and bamboozlement by their political class and agreed to ratify the identical Nice Treaty that they rejected last year by 54 per cent to 46 per cent."
He said the No side had done well to secure such a high vote against the forces of government throughout the EU and the print media.
Green Party TD Mr John Gormley reflected similar concerns: "There was, I think, a huge guilt trip, a certain intimidation - and a lot of money as well."
Meanwhile the former taoiseach Dr Garret FitzGerald, who campaigned for a Yes vote, said he was "overwhelmed" by the result.
"It shows if you run a good campaign it makes the difference. We had no campaign last time and I do think towards the end people were getting very fed up with the number of things being said by the No people which turned out to be untrue," he concluded.