Possibly a bit of a red herring here, but there is a version of 'The Three Ravens' communicated to Motherwell by Andrew Crawfurd and collected by him from Mary MacQueen (who had Ayrshire connections). Emily Lyle of the School of Scottish Studies in Edinburgh has published a two volume edition of Crawfurd's Collection which contains some tunes, and several songs that didn't reach Motherwell; she also instigated the recording of all the ballads associated with the said Mary MacQueen, sung by Jo Miller.
However, I have heard another tune than the Breton one to 'The Twa Corbies' -- it was sung by John Eaglesham (erstwhile singer and concertina player with The Clutha and, latterly, Stramash). He thought he had found it in a book somewhere, but couldn't recollect which one! It was similarly modal in nature.