The melody is most likely much more recent and much less traceable than the text. Breton music tends to use tunes interchangeably without naming them; this one sounds a lot like a lot of other Breton tunes.

This (from the sleeve notes?) implies it's a versified folk tale of mediaeval goliardic type but doesn't give a date:

http://claire.allenou.free.fr/Discographie/Albums/Urba.html