I play viols and do almost exclusively folk music on them. I play the bass, which is great for slower things like O'Carolan, but not quite the thing for jigs and reels (it can sound fine on these, especially at slower tempos). I played the treble for a while, mainly working on Irish dance music, and had some success, but the range of the treble is a little lower than the fiddle, and requires shifting up to the higher notes in many tunes. about 7 years ago I discovered the pardessus(fr. "above treble")which is like a treble with one high string added and the bass string removed, and that really is the ticket! you can see and hear it here:   http://www.youtube.com/watch?v=EbIb3QBpySI&feature=channel