Well you couldn't take the beat from the bodhran player, could you?
I looked up Tom Anderson's published version of Da Slockit Light (in "Ringing Strings"). No chords given, but there is a second fiddle part, "written by an American student in 1979". The chords you get from that fiddle accompaniment are, I think:
-|D - - -|D - - -|G - D -|G - A7 -|
D - - -|D - - -|G - A7 -|D - - :|
-|D - A -|G - D G|D - E -|A - - - |
D - A -|G - - -|G - A -|D - - - |
D - A -|G - - -|G - D -|G Em A D |
D - - -|Bb - D -|G - A7 -|D - - -||
So Bb it is, though I'd never have guessed it. Maybe the American student was copying a WillieJohnsonism?