Words for wolf in Celtic languages.

Proto-Celtic	*waylos = wolf
Middle Irish (Gaoidhealg)	fáel [faːi̯l] = wolf, howler
Irish (Gaeilge)	foal [fˠeːlˠ / fˠiːlˠ] = wolf
Scottish Gaelic (Gàidhlig)	foal [fɯːlˠ] = wolf, wild dog
Manx (Gaelg)	filliu = wolf

Etymology
From the Proto-Indo-European *waylos (wolf, howler) – this was apparently used instead of the usual PIE word for wolf, *wĺ̥kʷos, due to taboos [source].

Note: faol is an archaic and literary word for wolf in Irish and Scottish Gaelic. It does appear in the word faolchú (wild dog) though. More common words for wolf in Irish include mac tíre (“son of the land”) and madra alla (“wild dog”). In Scottish Gaelic a wolf is madadh-allaidh (“wild dog”), mac-tìre (“son of the land”) or faol-chù (“wild dog”).

Proto-Celtic	*bledyos = wolf
Old Irish (Goídelc)	bled = sea monster
Irish (Gaeilge)	bleidmhíol = monster, whale
Scottish Gaelic (Gàidhlig)	bleidh-mhial [ble viəlˠ] = sea monster (esp. whale – archaic)
Proto-Brythonic	*blėð = wolf
Middle Welsh (Kymraec)	bleidd = wolf
Welsh (Cymraeg)	blaidd [blai̯ð] = wolf
Old Cornish	bleit = wolf
Cornish (Kernewek)	bleydh [blɛɪð / bləɪð] = wolf
Old Breton	bleid = wolf
Breton (Brezhoneg)	bleiz [ˈblɛjs] = wolf

Etymology: unknown – probably borrowed from a non-Indo-European language [source].

Words marked with a * are reconstructions.

Sources: Wiktionary, Am Faclair Beag, teanglann.ie, On-Line Manx Dictionary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau