"Iun" / "Idunet" seems to mean "wilful, desire" in Old Breton, at least according to the Centre de Recherche Bretonne et Celtique:
http://www.wales.ac.uk/Resources/Documents/Research/BretonPatronymsBritishHeroicAge.pdf
Others seem to identify the "iun" element as being derived from "*ad-ioun" and compare it to Welsh "eiddunaw, addunaw" ("wish for, desire").
I can't vouch for any of those derivations at the moment, though.