The Goidelic (Irish and Scots Gaelic and Manx) and Brythonic (Welsh and Cornish) languages

Let's not forget Breton, also of the Brythonic branch. There is another branch, the Continental, which includes Gaulish, and perhaps the other Celtic languages in Spain and Northern Italy / Southern Switzerland (e.g., Lepontic).

As for fey being Celtic. Do you have the root? Or any reflexes in individual Celtic languages? Fay in the sense of "fairy" is via Middle English from Old French. Fey, in the sense of "fated to die, doomed", is from Old English fǣge. "Fate, doom, destiny" in Welsh is tynged.

Ceci n'est pas un seing.