Audiences watched Glen Hansard and Marketa Irglova fall in love on screen in the 2006 Irish charmer “Once.” What they didn’t know at the time was they were making beautiful music together off screen as well.

“The Swell Season,” the new documentary available today on DVD, catches Hansard and Irglova slowly, irreparably falling out of love while on their post-“Once” tour. The collapse is as delicate as one of Irglova’s ethereal harmonies. You have to squint to see the fissure developing between the duo. What’s missing are other elements to bridge the dramatic gap.

“The Swell Season” is stingy on showing the duo’s best musical moments, and there aren’t enough pop star theatrics to balance the sense of quiet doom blossoming between them. You won’t see any trashed hotel rooms here, but given the modest narrative in play you’ll almost wish you did.

The film’s story sneaks up on you all the same, something co-directors Nick August-Perna, Chris Dapkins and Carlo Mirabella-Davis clearly intended. But we never get close enough to the main players.

The team behind The Swell Season, the name the duo chose for their joint musical project, carry on matter of factly through their lengthy tour.

Marketa tells how she met Glen as a teen, but by the time she came of age she had fallen in love with him. One can question the details of their initial courtship, but it’s hard to argue when they take the stage together.

Fans ask them for hugs and autographs in between concert stops. Hansard obliges while Irglova bristles. The film catapulted Hansard into the spotlight after years of playing with his band, The Frames. So he stood ready to process the adulation. The much younger Irglova finds success unnerving. Why are these strangers asking to have their pictures taken with her? I’m not a celebrity, she says.

That’s merely the start of the couple’s woes.

The documentary, shot in lush black and white, captures how the two feed off each other creatively even while their relationship hits difficult patches. Their union made Hansard’s music blaze with possibility. He recalls writing entire songs within a few hours thanks to her inspiration and guidance.

The most effective moments come when Hansard sits down with his mother, a woman who can’t read enough headlines about her famous son. The singer’s father, a former boxer who turned to the bottle early in life, is too absorbed by his own demons to give Hansard much support.

It’s hard not to both applaud and curse the restraint shown throughout “The Swell Season.” Why the duo agreed to the documentary is another matter. They both appear intensely private people, a far cry from The Kardashian clan over at E!. But watching “The Swell Season” makes one appreciate the duo’s personal integrity as much as their music.

The DVD includes deleted scenes and extra concert footage from the duo’s tour.