Dan Tyminski Who is part of the band "Union Station" (Allison Krauss's band) was the person that did the singing for character that George Clooney played in "O" Brother Where Art Thou".
No that is not him singing
No, he did not sing in the movie. I am not really sure who it was but I loved it!
No, it was a famous bluegrass guy, but I can't remember his name. Mac Wiseman comes to mind, but I'm not that sure about it.