Old Irish word art comes from Proto-Indo-European *h₂ŕ̥tḱos (Bear.)

Detailed word origin of art

Dictionary entry	Language	Definition
*h₂ŕ̥tḱos	Proto-Indo-European (ine-pro)	Bear.
*artos	Proto-Celtic (cel-pro)	Bear (mammal).
art	Old Irish (sga)	Bear.