Hi Greig, I know Margaret's waltz was written by an Englishman but are you serioulsly suggesting that it is not a celtic melody.
Do you have to be born in a celtic country or have celtic genes to compose a celtic air.
It is probably one of the most popular tunes that kids in the Scottish Highlands learn to play and in my (non pedantic) book that makes it a celtic tune.
Incidently you call Shetlanders "Celtic"! Have you ween their flag - it's Scandinavian.