Lenny Abrahamson, the director of 'Adam Paul', has cast the Irish star to play the much-mourned Frank Sidebottom, alongside Domhnall Gleeson and, as a theremin virtuoso, Maggie Gyllenhaal, and they've been filming secretly in Co Wicklow.

Something unexpected is happening at a secret location in Wicklow. Negotiate your way past a vertiginous mountain pass, stumble across an impossibly idyllic babbling brook and, if you've got the right brook and the correct pass, you will encounter an incongruous crop of sub-Bavarian chalets. A bizarre party is lined up waving goodbye to unseen guests. One of their number, tall beneath a woolly Russian hat, looks a great deal like Maggie Gyllenhaal. Another is surely Domhnall Gleeson. A third wears a giant, bladder-shaped head with cold comical eyes. The head comes off. Good grief! It's Michael Fassbender.

We are on the set of the latest film from Lenny Abrahamson. It took five years for the Irish director, lionised for his debut, Adam Paul, to follow up the slippery, moving Garage. After performing strongly at the Irish box office, What Richard Did, his tale of tragic goings on among the Irish middle classes, went on to receive staggeringly positive reviews from British critics. Happily, it seems we will not have to wait quite so long for his fourth piece.

Based on a script by the journalist Jon Ronson and the busy screenwriter Peter Straughan, the baldly titled Frank traces its origins – distant origins, as we shall hear - to Ronson's relationship with the much-mourned postpunk entertainer Frank Sidebottom.

A creation of the late Chris Sievey, Frank, decked out in that scary papier-mache head, gained minor celebrity by singing such odd, unsettlingly naive tunes as Anarchy in Timperley and Xmas is Really Fantastic.

I have, however, barely set foot on the muddy track before it is explained that the film is no sort of Sidebottom biopic (if such a thing were even possible).

"The vague story is about a guy called John who comes across this eccentric American band," Ed Guiney, Abrahamson's inseparable producer, explains. "He then goes back to his normal life and is called to meet them at a gas station. He joins them, and they come here to record a weird album and later make their way to Austin."

Whereas Sievey made much of his northern English roots, this Frank is an American of the current avant-garde school. Sidebottom was a stage act. The disturbed hero of Abrahamson's film seems reluctant to ever remove his false head.

Hang on a moment. They've hired the world's hottest actor for a role that requires his face to be concealed throughout. This sounds properly weird.

During a break in filming, Fassbender, slim and graceful in a plain autumnal cardigan, makes his way towards an outlying chalet to explain himself.

"Lenny didn't have to talk me into it," he says. "I read the script and thought it was hilarious. It's so very different to everything else you read. Half of the time you can guess what will happen from page 10. That was definitely not the case with this."

So let's put this to bed. This is not the Frank Sidebottom story. "I have no idea," he says. "I knew nothing about him until I read the script. Then I began Googling stuff about him. I had no idea what he was about. Everything is in the script."

Distant background

Ronson was closely involved with Sidebottom. He briefly played with his band and, when Sievey died in poverty, helped to raise money for his funeral. Did Fassbender not ask Ronson about the film's distant background?

"No, I don't like to talk to writers about their writing," he says. "I really don't. I think that's a cheat. It's taking the easy route - getting the answers directly. They write what they write, and then it's handed over to me and the director and everybody else. The work then doesn't get set within one person's firm idea."

You could argue that Fassbender, raised in Kerry to a German dad, is one of those actors who really does justify the overused metaphor that links certain actors with chameleons. The intense, wasting protagonist of Hunger, Steve McQueen's study of Bobby Sands, came across as a very different figure from the withdrawn Rochester in Jane Eyre. The cold cyborg in Prometheus was something else again. Still, his face remains a valuable piece of cinematic real estate. He must have had reservations about covering it up.

"No, I am going to do this all the time from now on," he says with a laugh. "I am going to wear the head for everything. I love that idea. Ha ha! But, no, it really is part of the story."

We trump through a temporary hurricane to watch the completion of the scene. Somebody has mentioned that Gyllenhaal's family is visiting. Sure enough, her young daughter, a minute, near-perfect facsimile of mum, joins us to huddle around a monitor at the rear of the shack. Then Dad appears. It's the charismatic, currently heavily bearded actor Peter Sarsgaard.

The head is replaced on the willing star. Some more waving takes place.

Later, ploughing through a very decent lunch in the inevitable double-decker catering bus, Abrahamson explains how they tailored the scene to Fassbender's special abilities. The band (whose name is comically unpronounceable), having dissuaded a group of German tourists from evicting them from their recording space, are sending the travellers on their way. The opportunity to have Fassbender deliver a few lines of his second language was not to be overlooked.

to be continued...