Ireland's experience in Europe had been "overwhelmingly positive" and this would be reflected when people voted in the second Nice referendum, the Taoiseach, Mr Ahern, told a conference in Dublin Castle yesterday.
"It is rapidly becoming clear that the second referendum will extend far beyond the issue of Nice itself. Ireland's position in Europe, the contribution of Europe to Ireland's economic and social development and the enlargement of the Union, are all issues that are highly relevant," he told the European Foundation for the Improvement of Living and Working Conditions.
Mr Ahern said he was "a passionate believer" in a strong Ireland in a strong Europe. "We are at a historic crossroads in our national life.
"The forthcoming referendum will give the people an opportunity to demonstrate their belief in Europe and their wish to maintain Ireland's rightful place at the heart of Europe."
Ireland's future prosperity required us to stay at the heart of Europe, he said. "Ratifying the Nice Treaty is absolutely essential if we are to do so. Some time ago I challenged the No side to put the economic case for rejecting Nice. There was a deathly silence."
He said that 30 years ago some of the same voices advocating a No to Nice now were predicting doom and gloom if Ireland joined the European community.
"They entirely misunderstood the whole nature of the European project then. Some of these same voices are still predicting doom and demoralisation. Thirty years ago they were wrong. They continue to be wrong.
"What some of those who advocate a No vote forget, or ignore, is that the European Union is not just a legal entity. And it is not just an economic entity. It is also a political and human project. Goodwill and perception count for an enormous amount.
"Ireland has in the past benefited hugely from the goodwill of others. We have benefited from the perception, based on the facts, that we are a real player in the European Union."
Next week, the Dáil would be debating the Referendum Bill and "sometime later" a referendum to ratify the Treaty would be held. "From now on I, and every member of the Government, will be out there putting the case in favour of ratification," Mr Ahern said.
The Tánaiste and Minister for Enterprise, Trade and Employment, Ms Harney, told the conference that the No campaigners were offering "siren voices, wild-eyed warnings and red herrings".
The European political, social and cultural heritage was Irish as well as German, French, British or Italian.
"It is therefore a natural, historic and fully Irish progression that we should open the EU to more member-states, and by doing so, further develop our economy, our jobs and our political culture as Irish Europeans.
"We should stand by that tradition in voting Yes to the Nice Treaty, to take ourselves and Europe forward, to secure our own and our children's jobs and to keep Ireland open and vibrant.
"For the Irish people to take the advice of the No campaigners would be to disown and reject our Irish and European tradition, the roots of our culture and the means of achieving economic and social progress.
"It would be a profound reversal with direct, practical consequences for many people's jobs and livelihoods," Ms Harney said.
In a press statement, the Sinn Féin TD, Mr Aengus Ó Snodaigh, accused the Dublin Chamber of Commerce of "scaremongering" on the Treaty. He challenged the chamber to "produce evidence to back up their dire warnings of economic disaster for the city if the people vote No a second time".
He added: "Much the same threats were made against Denmark when they voted No to the Maastricht Treaty but not only are they not financially ruined, Denmark took over the presidency of the EU this summer."
The Independent MEP for Connacht-Ulster, Ms Dana Rosemary Scallon, said in a statement that "the Irish Government will not uphold Irish sovereignty and will bow to EU pressure for an EU constitution as the EU continues en route to an EU superstate".
She added: "The Taoiseach must clearly state in Europe that Ireland will not agree to an EU constitution and that we will not be part of any negotiations or talks leading to a Europeans superstate. Only the Irish people can decide on this issue."