The idea of a Bb chord occurring anywhere in a "traditional-type" tune played in D is truly bizarre.

I actually love this modulation, in its place, (my shorthand pet name for it is the "Honey Don't" chord change, after Carl Perkins' eponymous song which relies heavily on this particular modulation) and the other song which springs to mind which uses it is the Beatles' "I Will", right at the end.

There are a number of blues/ragtime songs sung by Blind Blake which use this change to good effect as well, but I am afraid you are never going to hear it in any melody remotely connected to the Scottish/Irish/Celtic tradition.

And Will Fly is totally correct about the F#, absolutely the right chord at that point in "Da Slockit Light".