This has been confusing me quite a Bit as Wales has been under British Rule the longest out of the Celts (except Britanny which is obviously under French Controll) but unlike Scotland or Ireland the Celtic Language is still pretty prominent in Wales and i want to ask why, why does Wales still speak a lot of Celtic unlike other Celtic Nations?
(Also ik that Celtic isnt really a Language but im using the word to just describe Celtic Languages in general)
I'm Welsh and wanted to find out too. This AskHistorian thread from a couple years is almost identical to yours, should help you out.. Author deleted their account so can't properly credit, but u/Typologyguy has a detailed answer for Ireland specifically. Sorry if this shouldn't be a top-level comment.