The Minister for Foreign Affairs, Mr Cowen, has rejected criticisms that the Government failed to look after national interests during recent negotiations on the European Constitution.
In what was being seen as a marker for the forthcoming debate on the proposed constitution, Mr Cowen also launched an attack on Sinn Féin's European policies, claiming they would alienate most Europeans.
Mr Cowen made his comments when he fielded questions from 18 politicians during a question and answer session at the National Forum on Europe.
During the session, Sinn Féin's MEP for Dublin, Ms Mary Lou McDonald, claimed that the new treaty "copperfastens what is called the democratic deficit".
"I would like to have been in a position to welcome the constitutional treaty, but on behalf of Sinn Féin I'm not in a position to do so," she said.
"It explicitly sets down the primacy of European Union law," she said. The treaty also "confirms the ongoing trend to centralisation of power at the level of the European Union".
It would undermine "the ability of citizens to influence key areas of public policy".
Ms McDonald also said the party should not be labelled as anti-European because of its opposition to the new constitution.
Mr Cowen, however, accused Sinn Féin of advocating a policy where each country would have a vote of equal importance, which would mean that Luxembourg, with 400,000 citizens, would have the same influence as Germany, with 80 million inhabitants.
It would create "the greatest alienation possible, if one were to adopt the Sinn Féin ideal", he said.
He said the treaty was the best one available, and that it protected Ireland's national interest in key areas.
"This treaty is the consensus that prevailed, but I can't accept the argument or contention that there was another treaty available, and I certainly would reject the idea that we don't defend our national interests."
The constitution allowed Ireland to retain its sovereignty in relation to tax matters, he said.
It also copperfastened Ireland's ability to follow a policy of military neutrality, he said.
Mr Cowen said Sinn Féin could not expect to avoid an anti-European label in the forthcoming debate on the constitution.
"Well, I mean, the democratic governments of 25 member-states supported it and [Sinn Féin's] position is 'when I oppose this treaty I'm not against the European project'. I think you're flying in the face of the intrinsic evidence which points to the contrary.
"If we're going to have a debate, we're going to have it on the basis of some people are for Europe, and some people are against it, we're not going have people placing an each-way bet."
Mr Cowen also rejected as "completely absurd" criticisms from other speakers, including Mr Roger Cole, of the Peace and Neutrality Alliance, and former Green MEP, Ms Patricia McKenna, that the treaty would help create a European military super-state.