Of course, some of the 'Welsh' tunes are also known elsewhere. I have a book of Welsh tunes which lists, for example, Mwmpwy Porteynon (Think I spelled that right, translated as Porteynon Whim). This tune, which I do play, turns out to be The Grand Hornpipe, recorded by Eliza Carthy (and probably others).

Galch (calch?)Gwyn Morgannwg, recorded by Calennig, is very similar to White Joke.

Ty Coch Caerdydd (Red House in Cardiff) is a variant of a long line of tunes which apparently goes back hundreds of years, and to Scotland.