Thanks, Bill. Let's hope that rubbish by Anon. is over now and we can return to real Celtic music.Music of the p-Celts. I have no information on any tune that might have been published in Brittany before the end of the 18th century, nor can I find in BUCEM any books of music published on the Isle of Man prior to 1800.
This leaves Welsh tunes as the only ones I know of. Edward Jones published 'Musical and Poetical Relicks of the Welsh Bards' in 1784, with an extended edition in 1794, with a second part in 1800. John Parry has a few Welsh tunes scattered around in some of his publications which are listed in BUCEM.
The 16th century tune "Sedany or Dargeson" may be a Welsh one. Sedany is a corruption of the feminine name Sidanen, and a ballad by Lodovick Lloyd of 1579 in praise of Queen Elizabeth calls for the tune as "Welshe Sydanen". (Internet broadside index ZN3235)