The upcoming Nice Treaty referendum will be the most important issue put before voters in Ireland's 30-year relationship with the EU, the Taoiseach, Mr Ahern, has declared.
Speaking near the end of the two-day Seville EU leaders' summit, Mr Ahern, who returns home with two declarations copperfastening Ireland's neutrality, said: "Let no one underestimate the seriousness of what is at stake.
"Thirty years ago, the people made a vital decision to join the European Community. Thirty years later on we are at a defining moment in our relationship with the European Union.
"We are appealing to people to consider this in all of its vital dimensions; economic, social, political and moral. We are confident that when all of them are considered, people will be in a position to ratify the treaty."
The Government's declaration on neutrality, and an accompanying one from all 15 leaders, will prove to a significant ingredient in the Yes campaign, he told journalists.
"The Seville declarations will bring clarity to the debate in Ireland and are a significant stepping stone to a greater understanding and to remove ambiguity in people's minds."
He said the 10 countries waiting to join the EU were anxiously awaiting the result. "They know that a failure by us to ratify the treaty will block their aspirations to entering."
Leaders from a number of the candidate countries have already visited Ireland to speak at the National Forum on Europe despite their own heavy work schedules at home.
"They have taken on the No side and, if I don't mind saying, buried the No side. People should go back and look at the papers of what they have said. They have left the No side speechless other than to go whingeing about their side of it," he told The Irish Times shortly before the EU leaders met with all the candidate countries.
He said some were now offering to help promote the Yes campaign. "We were not anxious the last time to march them in, but I can tell you this time they are very anxious to march in."
A second No vote would damage Ireland's EU standing and leave multinational companies based in Ireland "with no other alternative but to see where they should be".
Rejecting charges that the Government was prepared to scare the public into voting Yes, Mr Ahern replied: "I don't envisage that happening, and I don't want to be involved in scaremongering.
"I think that there were people on the No side who got away with hell and high water in the last campaign, and who were allowed to get away with it. And they are not going to be allowed to do it again."
Urging people to face facts, the Taoiseach said Ireland was an island heavily dependent on exports and foreign investment. "The alternative is to sideline ourselves. It is a simple enough proposition."
Instead of rejecting Nice, voters should look at the opportunities which will flow from the creation of a 25-strong union. "Of course, there are some concerns, but the opportunities outweigh all else.
"I have spent 15 years at European councils of one form or another and I have seen us come from 18 per cent unemployment, a national debt like Nigeria's and a future that was going absolutely nowhere to being a really dynamic country."
The "only logical conclusion than any sane person" can draw is that progress has been based upon EU membership. "If I was here as a Taoiseach calling for anything other than full involvement in the EU into the future I would be wholly irresponsible."
Questioned about farmers' lack of support for the Yes campaign last time, Mr Ahern warned that the Government's ability to safeguard agricultural interests would be harmed by another rejection.
"We achieved a great deal at Berlin on agriculture. It comes up for review next year. I know which way I would like to be coming into it.
"I would not like to be coming out here with myself handcuffed. You can imagine how far I would get."
He rejected suggestions that the Government should have demanded concessions, including a guarantee that Ireland will have a European Commissioner, once the EU exceeds 27 countries.
"We got a good deal on the issues of concern to us in Nice. We were very happy. The small countries achieved what we wanted to achieve.
"I would be glad to campaign on the issues that were agreed at Nice," he said.
However, he again acknowledged that mistakes were made by the Yes campaign, including the political parties, in the last referendum. "We never managed to capture the imagination of the people. That is a problem.
"People got influenced by other things. A lot of them were influenced by a costly campaign that had posters up around the place. We have to address that as best we can."