When a trio of young American directors set out to make a documentary on The Swell Season – the duo comprising Glen Hansard and Markéta Irglová – they found themselves saddled with a peculiar problem. The film concerns itself with the relationship between beardy Hansard, a Dublin musician, and shy Irglová, a younger Czech singer. Didn't John Carney already make that film in Once?

As things worked out, the true-life version played a little like Carney's film in reverse. Shortly after winning that famous Oscar for best original song in 2008, the singers embarked on a hectic tour of the United States. The Swell Season (as the film is baldly titled) chronicles the slow, edgy break-up of their relationship and their complex engagements with cult celebrity.

"It was an interesting endeavour to embark on," Carlo Mirabella-Davis, one of the three directors, explains in a massed phone-call from New York. "In the fictional film they had essentially played themselves. And there was a tension between those fictional characters and the people they really were. That intrigued us. But as we progressed, we focussed less on Once. When we met Glen's family we became interested in themes such as the burden of dreams."

Mirabella-Davis first met Hansard when he was teaching at the New York Film Academy. A man of no small ambition, Hansard – also front-man of the legendary Frames, of course – signed on for a course just after he won the Oscar.

“After the class ended, he approached me,” Mirabella-Davis says. “He explained that he was going on a long, extended tour of America and he was interested in making a film of it. He wanted to know if I could recommend any directors and I mentioned Nick and Chris.”

"Nick and Chris" were Nick August-Perna and Chris Dapkins. Together the three men have delivered a rather lovely study of a duo in a state of dangerous flux. Shot in gorgeous monochrome, the picture follows The Swell Season about large venues as they come to terms with a sudden surge in popularity. Hansard has already had some experience, of course. But Irglová seems increasingly fazed by the attention. At every stage door, young people with camera-phones demand digital souvenirs. The cliche "rabbit in the headlights" is close to unavoidable.

“It’s not inaccurate to describe it that way,” Dapkins says. “She’s an unbelievably resilient person. Given what was coming her way and faced with the need to be a celebrity when she was used to playing these smaller festivals, it really did have the effect of catching her like a rabbit in the headlights. By the end of the film she’s a stronger individual.”

So given that Hansard, in a sense, commissioned the film, it would be interesting to hear what contribution he had to the final cut. You wouldn't call The Swell Season any sort of hagiography. But devoted Hansardians (of which there are quite a few) are unlikely to be left in any state of outrage by the finished project. We travel home to Ireland and encounter his troubled father and his indomitable mother. He talks us though early days at school. The slow break-up with Irglová is detailed through tense conversations rather than stand-up fights. He's not David St Hubbins and she's certainly not Jeanine Pettibone.

“We screened the film for him several times,” August-Perna says. “To be honest it was more out of respect than looking for feedback. From the very beginning, he envisioned a tour film. At the beginning we showed him some footage of them and they suddenly realised we were filming things they hadn’t expected. But they were excited and they handed over the reins.”

Some of the recorded conversations are distinctly odd. In such documentaries, intimate chats tend to be snatched in noisy, chaotic surroundings that allow the participants to let their guards slip. It’s not always like that here. On one occasion, Hansard and Irglová are sitting in an outdoor café, discussing their personal feelings while a camera pans calmly from one face to the other. You’d think they’d ask the documentarian to go elsewhere before having such a conversation. Would you not?

"We want to make this very clear," says Mirabella-Davis in a voice that invites no contradiction. "There was no direction of them in the making of this movie. We were very strict in the making of this film. We were going for straight cinema verité. We barely opened our mouths when we were around them. Over the course of the year, we became invisible to them. As for the scene in the café, they might have realised the camera was on. But Chris especially developed a technique of making the camera unobtrusive."

The directors were faced with a tricky decision when releasing the film in the US. Glen’s mum, who speaks with a pronounced, but perfectly lucid, Dublin accent, was put before American viewers with subtitles. Oh they’ll be snorting about it in the pubs of the capital.

“To be honest, it premiered here in the States and we had a bunch of press screenings,” Nick says. “There was a consensus that subtitles would help in this country. My general rule is that if there is any doubt about what is being said you should subtitle because you don’t want to lose the drama. It may seem like overkill in Ireland. But we felt the conversation with the parents was so important we didn’t want to miss a word.”

At any rate, the film ends up offering a largely flattering portrayal of the home sod. Glen gets to stroll around various ancient ruins. His parents prove to be wise sages. The American trio have done the nation proud.

“We showed up and we instantly fell in love with the country,” Mirabella-Davis says. “And what I immediately noticed was the appreciation for art, films and music was more potent than in any other country I’d been to. Everybody cares so deeply. Making art is like a national religion.”

Yeah, it’s not such a bad old place.

yyy The Swell Season opens on December 6th