Galicia, of course. I would have forgotten.
Considerring the Alps, Rhaetians don´t seem to be Celtic, if some language were to survive in the Alps, I can mostly likely see it in upper Carinthia, maybe some parts of Styria, or possibly the Valais region of Switzerland and the Lake Luzerne area. Although these areas might be refuge zones, you would also need a relativiely dense and numerous population there.
I think the one thing that fastened the disapearance of Celtic languages in the Continent was the Latin mass; either make those areas not go Christian or allow for a Celtic translation of the bible and Missal.