Words for hen in Celtic languages.

Proto-Celtic	*kerkā = hen
Old Irish (Goídelc)	cerc [kʲerk] = hen, female bird
Irish (Gaeilge)	cearc [kaɾˠk] = hen, female bird
Scottish Gaelic (Gàidhlig)	cearc [kʲɛrxg] = hen, chicken; poultry; female crab or lobster
Manx (Gaelg)	kiark = fowl, hen, hen bird, female bird

Etymology: from the Proto-Indo-European *kerkos [source].

Proto-Celtic	*yarā = chicken
Old Irish (Goídelc)	eirín = pullet, young hen
Irish (Gaeilge)	eireog = pullet, chicken; young girl, tiny woman
Scottish Gaelic (Gàidhlig)	eireag [erʲag] = pullet, young chick(en)
Manx (Gaelg)	earrag = pullet, poult
Old Welsh	iar = hen
Welsh (Cymraeg)	iâr [jaːr] = hen, hen-bird ieir = domestic fowls, chickens
Cornish (Kernewek)	yar = chicken, hen
Breton (Brezhoneg)	yar = chicken, hen

Etymology: unknown

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