Since this thread has re-appeared after a lull of several years, I'll add a couple of useful resources that have appeared in the meantime:

'A Manx Note Book': includes several song collections and related works:

http://www.isle-of-man.com/manxnotebook/


'Son ha Ton: chansons traditionelles bretonnes' (with staff notation, and often translations into French; includes material from the Barzaz Breiz):

http://perso.wanadoo.fr/per.kentel/

Llanerch Publishers didn't close down as had been feared. It still operates, though now as Llanerch Press:

http://www.llanerchpress.com/