Reaction: The Taoiseach expressed bewilderment and confusion at the latest Stormont spy revelations at an early morning press conference in Brussels on Saturday.
Speaking to reporters at the conclusion of the EU budget negotiations, Mr Ahern said he had always been "totally sceptical" about the Stormontgate affair - the police raid on Stormont which was reported to have discovered a major IRA operation involving spying on other parties, and the collection of information on the security forces.
The affair was the final trigger for the collapse of the North's political institutions in late 2002, when Ulster Unionists said they could not continue to share power with Sinn Féin in such circumstances.
"Remember what happened," the Taoiseach said. "That film piece that they still show of all of the storm troopers charging up the stairs with heavy armoury to collect a few files, and to arrest a few people. It never added up.
"Last week when I asked Tony Blair why did it [ the trial of the three men charged in the wake of the raid] collapse, he couldn't understand it either. So I've never been happy with it. And it created enormous grief for us, because the whole institutions were brought down, it created huge difficulties for David Trimble."
While expressing lack of comprehension, he repeatedly stressed the political significance of the Stormontgate affair, and the way news of it had emerged. "This was a huge case. It doesn't get bigger than bringing down democratically-elected institutions that people voted for. What this is about I just don't know.
"Now we are asked to believe that the person Sinn Féin had in there looking after the administration was also in there by the British security. So he had the confidence of Sinn Féin and he had the confidence of British security to be in a key position that ultimately brought down the whole institutions. It stretches my imagination.
"Stormontgate never added up to me. A large number of police and huge armaments, storming in, to collect a few clerks and a few files and the TV was in first."
At his press conference shortly before 4am at the end of the EU budget negotiations in Brussels, Mr Ahern insisted he did not have any insight into what had transpired during the day. "All I know is that the chap who is running the Sinn Féin office up there turned out to be a spy."
From a Sinn Féin point of view the situation was beyond his comprehension as well. "On the face of it what we are now being told is the person who was the head of the administration, Donaldson, who we are aware of for a long time, that he was a spy.
"When they raided his house at that time, he opened the door and he happened to have a whole lot of grenades or something in a box under the telephone table. It never made sense."
The Taoiseach said that after the police raid on Stormont, Sinn Féin said they had carried out an internal investigation. They "carried out a whole detailed examination and they were quite happy that there was no problem.
"But it's kind of a bit confusing if they checked it out that time that they didn't find out when they were checking it all out that the fella they were checking out" was a British spy.
"We always felt that this thing didn't make any sense. Sinn Féin always protested their innocence on it and investigated it. But I think it's a fair question to ask, if they investigated so much that they didn't uncover that their key guy was a spy, so I don't know what happened inside but they must be very shattered about it, too. They are fairly secretive and fairly tight and they normally have their key people in key places, and to think that their head of administration in Stormont turns out to be a long-term spy."
He also questioned the behaviour of Denis Donaldson, after he had been "outed" as a spy. "If you were a kind of secret spy, you normally don't try to do interviews as soon as you are uncovered."
I don't know, I just don't know." He said he was confused about the fact that one minute Mr Donaldson was being told by the security forces that his life was in danger "and the next minute he's coming down to RTÉ . . . This spy drove from Belfast to Dublin to be interviewed by Charlie Bird. He's obviously stopped being a spy. So it sounds extraordinary."
He said the security forces had initially said there was "irrefutable evidence" of a spy ring at Stormont. "When I asked Tony Blair, having waited for three years for what was irrefutable evidence, he had absolutely no detail on it.
"This wasn't just another case, this wasn't just another investigation, another prosecution. This was an action that took place by the security forces where they said when they had collected the files and all of their stormtroopers - and a few secretaries - that this huge evidence was going to really prove everything, and the consequence of that was that it brought down the institutions. And it has caused us huge grief.
"And then, I don't know anything about it, the British Prime Minister didn't know anything about it, and now tonight we find out . . . and now we are told that the person at the centre of this from Sinn Féin point of view, even though they investigated it, they now uncover that this guy was a spy. But I'd like to hear all sides of it."
Mr Ahern stressed several times his belief that the British prime minister did not know the detail of what had happened.
"I don't think Tony Blair knew. I think that he didn't know. It's fairly bizarre and I can't read it."
He said he would not attempt to read it "until we just see what the circumstances are . . . It is a confusing state of affairs and if all of this proves to be true it would be a serious concern but we have to wait and see what really is the position."