Here is a link to the Breton lament "An Aotrou Nann" that may have inspired "Le Roi Renaud" (unless it went the other way round...):
http://chrsouchon.free.fr/barzaze.htm then click on the third link from top of the list: "Sir Nann".

The thumbnail Breton flag is a clickable link to the Breton text of the song.