The Taoiseach believes the Sinn Féin leadership was aware of a number of operations carried out by the IRA last year, including the Belfast bank robbery, while in negotiations with the Government.
Mr Ahern made the claim in the Dáil today based on briefings he received in London yesterday from PSNI Chief Constable Hugh Orde and Garda Commissioner Noel Conroy.
"They believed that a number of operations that took place during 2004 - not just the Northern Bank robbery - were the work of the IRA, had sanction from the Army Council and would have been known to the political leadership of the IRA," Mr Ahern said.
He said there could be "no possibility" of building trust and confidence in the peace process in the coming months unless the two governments are given "clear positions" on the issues of criminality and paramilitarism.
Mr Ahern was answering a question on his meeting yesterday with the British Prime Minister, Mr Blair, and with the two police chiefs in Downing Street.
Mr Ahern said a great deal of work had been put in on the issues and on the joint declaration last year, but that there was "no possibility of being able to get the trust back" if there were no answers forthcoming on criminality and paramilitarism.
"If we do not get clear positions on these, there's no possibility of us in the months ahead being able to build up trust and confidence," Mr Ahern said.
There was no possibility of getting anywhere until those questions were answered, he added.
The Fine Gael leader, Mr Enda Kenny, said it was "pretty obvious" that Sinn Féin had been negotiating with the Irish and British governments over a period "in very bad faith" and that they had clearly destroyed the level of trust that had been built up by "serious breaches of faith".
That showed the "level of folly" by the Government in putting the early release of the killers of Garda Jerry McCabe on the table, Mr Kenny added.
Mr Ahern said he accepted what Mr Kenny said "in relation to what was going on in the negotiations".
But he said "that was then and this is now and things move on". What the governments were now trying to do was to re-establish the institutions in Northern Ireland and to get them working on a cross-party basis. That could not be done until they had answers to those outstanding issues.
He said the overall objective now is full implementation of the Belfast Agreement.