The Soggy Bottom Boys are fictitious. The music and vocals behind George Clooney and friends were done by session musicians. That doesn't make it any less excellent.
The song was originally recorded in 1913 by blind fiddler Dick Burnett, as "Farewell Song," but it is unclear whether he wrote it or if it predated him. It has been recorded dozens of times, by such artists as Bob Dylan and Rod Stewart.
This version, from the movie "Oh Brother Where Art Thou", is a faithful bluegrass cover version of the song, and it works as such. This pivotal scene from the movie is a tremendous performance of this song that I truly hope you enjoy.