The song was recorded in E Dulenn (In Dublin) album. From there, switch to the much longer French entry and put the whole thing in a translator if you don't know French.

You have a Wiki article in French that Google translator translates rather well. The first paragraph translates as "Spered hollvedel ("Universal Spirit") is the arrangement of a traditional Breton song by Alan Stivell. The music comes from a tune to hymns among which are the one for Holy Mary of Rostrenen (Itron Varia Rostren) and for Sainte-Anne by Jeff Le Penven (Kantig Santez Anna)."
On this Spered hollevedel Wiki page, scroll down to the notes and links, the note 1 leads you to this page then click on the link "sellet gwerhiez vari", there's a sheet music. The note 5 leads you to this page, click on the "Itron Varia Rostren" link, there's a sheet music.