During the filming of the opening scene, because the scene was shot with long lenses placing the crew far away, and without informing the public, who would be crossing through the scene, a bystander attempting to be a hero accidentally injured the thief as he was running away by kneeing him in the groin.
Bob Dylan was such a big fan of the film that he arranged to have the two leads, Glen Hansard and Markéta Irglová, open for him on part of his world tour. Hansard and Irglová also covered Dylan's song "You Ain't Goin' Nowhere" for the I'm Not There. (2007).
The Dublin street scenes were recorded without permits so a long lens was used. Many passers-by didn't even realize that they were being filmed. The long lens also helped the non-professional actors relax and forget that they were on camera.
The lead role was originally intended for Cillian Murphy. He turned it down however because he was unsure about acting opposite a non-professional like Markéta Irglová (who was only 17 at the time) and also because he wasn't convinced his singing would do justice to Glen Hansard's songs. When Murphy pulled out, so did most of the money.
The title refers to the many very talented artists that John Carney knew who put off their career by saying "once" they get this and that sorted out, but never succeed because they've put it off too long. This describes the lead character, The Guy.
The song "Broken Hearted Hoover Fixer Sucker Guy" was filmed during a spontaneous moment during filming. It was never meant to make it into the film but the director liked it so much he kept it in much to the amusement of Glen Hansard who plays "The Guy".
When John Carney asked Glen Hansard to take on the lead, the singer was initially reluctant as he'd never done much in the way of acting, aside from one small supporting role in The Commitments (1991). He changed his mind when Carney assured him that the film would be low budget and very intimate.
Spent years in development with the Irish Film Board. During some of this time, the board had no chief executive for about six months and it was then that the film was given the go-ahead by a lower level executive on the proviso that the film be made for 150,000 euros.
The trivia item below may give away important plot points.
There is a moment when the guy asks the girl whether she loves her husband. She responds, "No. I love you." However, her response is in unsubtitled Czech, so the man does not understand her - nor do audience members who don't know the language.