ooh, doing a similar exercise here! - though unfortunately neither Irish or Welsh have anything to do with Breton (Cornish does though!)
I am afraid Fiona or anything "fion" is out (fion = arse in French slang), Saul is pronounced like the willow tree (not a nice thing in France)
Here is what we have collected so far (bit of a mix bag for French names):
Brittany:
For girls: Gwen (and derivatives: Gwenaelle, Gwenola…), Enora, Erell, Yuna (or Youenna, Youna), Morwenn/Morwenna, Sterenn (= "star"), Aziliz (= Cecily), Lena (= Helen), Corentina (possible nn Tina), Mariwenn, Rhiannon would also work.
For boys: Gwendal, Gweltaz, Alan, Armel, Malo (v popular in France at the moment), Erwan/Ewen, Yann (= Ian), Yves (= Ivo), Youenn (pr "Yoon"), Mael, Tristan
France:
Girls: Louise, Anna, Anouk, Charlotte, Juliette, Chloe, Alice/Alix, Judith, Madeleine, Gabrielle, Raphaelle, Laure, Rose, Marianne, Astrid, Zoe, Constance, Alexandra, Josephine, Faustine, Eva. Depends on your naming style here!
I would avoid Erin here (sounds too much like "urine" for it to work here)
Boys: much harder - Paul, Louis, Amaury (pr Amori), Baptiste, Samuel, Henry, Thibault (pr Teebo), François (Franswa), Rémi, Antoine
Insane amount of Breton names listed here: fr.wikipedia.org/wiki/Liste_de_prénoms_bretons
Classy but trendy French names: www.lefigaro.fr/assets/carnet/Carnet%20Prenoms%202014.pdf
good luck!