A row over the qualified majority voting in the Council of Ministers could prove a major stumbling block at the EU leaders' summit this weekend to finalise a new constitution.
The Brussels summit aims to agree the new constitution for an enlarged EU, but the Taoiseach, Mr Ahern, told the Dáil that there were a number of "red line" issues, crucial to individual member-states, on which countries had not changed their minds or progressed.
He believed the debate on qualified majority voting in the Council was a "potentially difficult area and I do not see a resolution at this stage".
There "has been no movement on the issue of the weight of votes, which pitches France and Germany against Poland and Spain".
The French and Germans favour a voting system that ensures that issues are backed by countries representing 60 per cent of the EU's population. France and Germany account for one-third of Europe's population.The Spanish and Polish favour the complex system agreed during the Nice Treaty.
The Taoiseach said he did not believe the changes were "enormously important", but Spain and Poland had a different perspective and were in a strong position post-Nice and that had "enormous political repercussions".
The Labour leader, Mr Pat Rabbitte, had asked if the summit would agree the constitution or would the Taoiseach be "carrying the torch into the Irish presidency".
Mr Ahern said there was an "overwhelming view" that they should attempt to "complete matters" this weekend, but he believed that "we are in for a long weekend. I will be lucky to be back by this time next week".
The Green Party's foreign affairs spokesman, Mr John Gormley, called on the Taoiseach to guarantee that Ireland would not become involved in either "structured" or "closer" co-operation on security and defence issues without putting it to a referendum first.
The Taoiseach replied that Ireland's commitment to "enhancing military capabilities is to ensure that the EU can effectively undertake humanitarian crisis management operations".
Mr Gormley "wants me to say this is outside crisis management. It is not. The position is that it is within the Petersberg Tasks."
However, the Fine Gael leader, Mr Enda Kenny, said that Ireland should be involved in the discussions to shape Europe's defence strategy, rather that "having it imposed from outside".
The Taoiseach said the EU Presidency's approach "would commit members-states to come to the aid of a member-state in the event of an armed attack, using all means in their power".
Mr Ahern said Ireland could not enter into such an obligation, and with Finland, Sweden and Austria had sent a joint letter to the presidency proposing an amendment to respect the position of neutral and non-aligned countries.
