There are quite a few Scottish tunes in flat keys (usually covering a two-octave range up from B flat below middle C) from the years 1780-1830. My guess on the reason for these is that they were meant to be played on the harp as well as the fiddle or piano, and the harp sounds better in flat keys: with the levers up you get a greater length of string vibrating. Some of these tunes go beyond that range, in which case you probably can't do any transposition without either falling off the bottom of the fiddle's range or driving it up beyond first position (even more of a no-no for less advanced fiddlers than weird keys). Look at Nathaniel Gow's "Mr Ronald Crawford" for an example (gorgeous tune in F minor, though it's doable on the three-flat clarsach because the D flat doesn't occur).

Anyway, with Scottish tunes (and to some extent Irish ones) the notion of the "right key" for each tune is so entrenched that if you expect to be playing with anyone else you have to stick to the key the tune was first published in.