I don't know if you've already birded that part of France but from a Mayo birder's point of view, and even without transport, there should be plenty of nice stuff to keep you occupied: Firecrests, Bluethroat, Nightjar, Dartford Warbler, Savi's, a few raptors, possibility of something interesting wader-wise at le Teich (
http://www.reserve-ornithologique-du-teich.com/en), Sardinian Warbler is spreading northwards from the Basque country and the bay itself could turn up a good bird in June. Lots of good habitats surrounding the numerous campsites etc so worth exploring. Good luck and drop me a line if you need more info.
Eugene