For those not familiar with the famous Sixties spy series Mission: Impossible, it usually started with a self-destructive message with the mission in question - highly dangerous, usually glamorous and, er, impossible - being described on tape to a nervous intelligence agent. Then the tape machine went up in smoke.

Now the cult TV series about the CIA has been remade as a Hollywood blockbuster, featuring the gleaming smile of Tom Cruise. And the mission of providing a Nineties update of its famous theme tune - dum, dum, dum, dum, dah, dah - or something like that - has been accepted by these rather less clean-cut agents Adam Clayton and Larry Mullen.

At great risk to life and limb (and after several dry martinis), I managed to trace the agent whose code name is "bassman" and wring a confession out of him. The offer to do the theme came from Paramount Pictures last year, just as U2 were reassembling after their long break, he told me.

"It seemed like the last thing the band wanted to do, as we were about to start focusing in on our own music and start scheduling our own record. We said no, because these things tend to take up more time than you could imagine. At the same time, everyone was excited about this great tune.

"Then I thought about it. There's always a bit of hanging around while we make a U2 record, and it is an instrumental, so maybe it was something Larry and I could so. So we went ahead."

Paramount were lucky to get their men. Film company executives all over the world would love to use an association with U2 to boost their film's chances. "It's very fashionable for movies to try and get a big name, but usually it's got nothing to do with the quality of the work or the relationship between the music and the film," said Adam, from his secret hideaway (otherwise known as his own front room).

"I was aware of that, but Mission: Impossible is such a great tune that we thought we'd try to do an updated version, and think of it more as something that would work in clubs rather than something that would promote a movie."

Members of U2 have taken part in a long list of collaborations. Larry and Adam worked together with Daniel Lanois and Nanci Griffiths, for example -- so how do they decide which of the many offers to accept?

"Generally you work from a priority list. When we're having a band meeting there is an agenda there that somebody else has already decided upon. That's what management does - goes through things and says, 'This is worth spending time on in a meeting.' It's really a question of how much time the band has, what the goals are for the next two or so years, and whether it fits into those inspirations or not."

So had the original series meant anything to him? "In nostalgia terms, yes. I remember seeing it...I don't know what age I was. I must have been 10 or 15. America, to a kid at that age living in suburban Dublin, was a very exciting place. To some extent, maybe the influence of American TV in Ireland had an effect on one's future and visions and dreams. We didn't even have BBC television, so we didn't have that English influence then."

The Theme From Mission: Impossible, written by Lalo Schifrin, has become one of those classic signature tunes, often used to signal glamour, danger and excitement. So when Adam and Larry sat down with the tune, as it stood, where did they go from there?

"You say, 'okay, who's going to produce and engineer? Do we need extra musicians?' You start refining it that way. It's not that complicated if the time exists. It's different than when somebody comes up to you and says, 'we need a theme tune and it can be whatever you want.' If you're re-recording a song it's really a question of getting the right people around to work with, and getting good performances."

Mother Records released a single of the updated theme in May, featuring two versions that are very different from one another. "Technically speaking, the original score was based on blues, which was what bands in the Sixties had been doing," said Adam. "But in order to make it a little bit different, he switched the time signature to 5/4, which is kind of Sting territory. It changes the emphasis of the beat and makes it very difficult to dance to. He was able to smooth it over using South American rhythms.

"That was successful, but it wasn't going to work in a club situation. So now we did another version in 4/4 time, which is fudging the actual tune. People don't quite know what's different about it, but one you can dance to and one you can't.

"We started work on the theme independently because we were in two different locations. And then we presented both to Paramount, hoping that they would make the decision easier by saying, 'Okay, this isn't the one we would like you to finish out.' They came back and said, 'Actually, we would like you to finish them both out.' It was at that point that we both were in the same location and we finished both tracks together."

Some of the final recording was done in Dublin, some with an orchestra in London and some in New York with an American percussion player. Both appear on the single along with remixes by Dave Clark, Junior Vasquez and jungle guru Goldie. They are also on a release by Mother, which features songs by Massive Attack, Pulp, Bjork and Gavin Friday.

The film, still being completed when we spoke, was partly shot in Prague, capital of the Czech Republic, and at Pinewood Studios in London, where a scene was shot involving a helicopter chasing a train through the Channel Tunnel. Press reports suggested on-set rows would cause the film to self-destruct, but it was due for a July opening. Adam's interest in the avant garde is well known, so I asked him how he would feel if the film turned out to be a turkey?

"Hmmm," he pondered. "Artistically or commercially? Artistically it is what it is: a Hollywood blockbuster action movie. They haven't concealed that from us. I'm sure it will be very good for what it is.

"We were only interested in the music, which existed beforehand. I don't want to bite the hand that feeds, but I'm not particularly interested in big Hollywood movies. Some of them are good, but usually, they're in an emotional and creative terrain that is not much to do with what turns me on as an artist.

"But it was an opportunity for Larry and myself to work on a great instrumental and update it. That was the reason we were involved. The agenda of Paramount Pictures or Tom Cruise was not something we were prepared to take on."