Words for squirrel in Celtic languages.

Proto-Celtic	wiweros = squirrel
Old Irish (Goídelc)	íaru = squirrel
Irish (Gaeilge)	iora = squirrel
Scottish Gaelic (Gàidhlig)	feòrag [fjɔːrag] = squirrel
Manx (Gaelg)	fiorag = squirrel
Proto-Brythonic	gwɨwer = squirrel
Middle Welsh (Kymraec)	gwiwair, gwiweir = squirrel
Welsh (Cymraeg)	gwiwer [ˈɡwɪu̯.ɛr / ˈɡwɪu̯.ar] = squirrel
Cornish (Kernewek)	gwiwer = squirrel
Breton (Brezhoneg)	gwiñver = squirrel

Etymology: from the Proto-Indo-European *wer- (squirrel, stoat) [source].

In Manx a squirrel is also a roddan biljagh (“tree rat”).

Words marked with a * are reconstructions.

Sources: Wiktionary, Am Faclair Beag, MacBain’s Dictionary, In Dúil Bélrai English – Old-Irish Glossary, teanglann.ie, On-Line Manx Dictionary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionnaire Favereau

Here’s a tune I wrote called The Scampering Squirrels / Y Gwiwerod sy’n Prancio: