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 film I'm Not There (2007).
Steven Spielberg was a big fan of the film. He said "A little movie called Once (2007) gave me enough inspiration to last the rest of the year."
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.
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.
The most expensive part of the entire shoot was the hiring of the crane for the final sweeping shot which cost £2,500.