from http://abqjournal.com/venue/personalities/126081person12-26-03.htm :

"Because his musical skills were limited, most of the music heard in the film was actually performed by a professional fiddler named Stuart Duncan and a vocalist named Tim Erickson and dubbed in over his performance.
    "We recorded the tracks in Nashville before filming," Gleeson says. "They mixed me in a bit, but most of what you hear is Stuart and Tim."