They associate the serpent with ... well, the closest equivalent might be Bres, Balor or Crom Dub, Crom Cruach... the thing is that I don't really remember our myths having a 'devil,' some great evil who sought to corrupt mankind and turn them towards darkness. The Fomori were bad, but they weren't that nasty in the tales.

Catholics obsess over temptation, and the serpent, the devil, was the one that started all of that. It's... well, I only know it so well. Sorry. I never looked into my father's mythology that deeply.