Jim McLean

I have traced this tune family back as far as an 11th century Gregorian chant "En Gaudeat"

Tantalising. Have you made your research available anywhere? I've been looking into this in a vague way for ages starting from the Cuckko's Nest/Jacky Tar side, a connection which the Fiddler's Companion doesn't seem to have made.

Another member of the family is King of the Fairies which came as a surpriose to me when Jack Campin pointed it out.