Fine Gael and Labour have demanded that the Government postpone the referendum on the Nice Treaty.
The Fine Gael spokesman on Foreign Affairs, Mr Jim O'Keeffe, said that while his party supported the referendum, its passage by the people could not be taken for granted.
He added that the date proposed for the referendum, May 31st, displayed a total disregard for the foot-and-mouth crisis in the State.
"Also, I think the Government is displaying a complete disregard for the democratic process by only allowing eight weeks between the publication of the White Paper on March 29th and the proposed referendum.
"That is a further compelling reason, in my view, for the postponement of the referendum until the autumn."
The Labour leader, Mr Ruairi Quinn, repeated his call to the Taoiseach to postpone the referendum.
"Debate about this treaty, and the realities Ireland faces, was non-existent before the negotiations began."
Mr Quinn said there were elements of the Nice Treaty which he believed could be better. It was true, too, that there were some issues which could be revisited.
"But it is also true that this treaty must be accepted for better or for worse as it is. It cannot be renegotiated. We cannot opt out of the core decision-making process of the Union.
"It is not a situation comparable to the one the Danes found themselves in after Maastricht. And in that light, it would be a travesty for Ireland as a postcolonial country to deny the benefits of EU membership to other post-colonial countries in Eastern Europe."
Earlier, the Minister for Foreign Affairs expressed confidence that the treaty would be adopted.
Mr Cowen said that being at the heart of Europe had given Ireland the opportunity to bring its distinctive voice to a wider audience.
"In return, we have opened ourselves to new currents and influences, which have both broadened our experience and enriched our national life. Nobody realises this more than our present generation of young people."
Mr Cowen was introducing the 24th Amendment of the Constitution Bill, 2001, seeking agreement in a constitutional referendum to the changes necessary to allow the State to ratify the Treaty, which was signed in February.
Mr John Gormley (Green Party, Dublin South East) said his party would be urging a No vote.
"We regard the treaty as a further attack on democracy, a treaty that moves decision-making further away from the Irish people, further into the hands of bureaucrats, ministers and lobbyists.
"We've now less democracy, less accountability, Ireland has lost its veto in over 30 new areas. We have lost our right to a commissioner, our voting strength in the crucial Council of Ministers, and we have lost three of our 15 MEPs."