Words for gods, deities & days in Celtic languages.

The Gallo-Roman Horse Goddess Epona

Proto-Celtic	*dēwos = god, deity, day *dēwā = goddess
Gaulish	deuognata, teuoxtonio-, dēuos, dēwos, dēvona = god
Celtiberian	teiuoreikis, deobriga = god
Old Irish (Goídelc)	día [dʲiːa̯] = god bandía = goddess díadae, deoda = godly, divine, holy deacht = godhead
Middle Irish (Gaoidhealg)	día, dea, déa, de, dee = god, goddess, supernatural being, object of worship deacht, déacht = godhead, divity díadae, díade, diadu = divine, godly, holy
Irish (Gaeilge)	dia [dʲiə] = god, deity bandia = goddess diaga = divine, godly, godlike, sacred, holy diagacht = divinity, godhood, godliness, piety, theology diagaigh = to deify, apotheosize diagaire = divine, theologian diaganta = godly, pious dia-aithis = blasphemy
Scottish Gaelic (Gàidhlig)	dia [dʲiə] = god, God ban-dia = goddess diadhachd [dʲiə.əxg] = deity, divinity, godhead diadhaidh [dʲiə.ɪ] = divine, godly, pious diadhaireachd [dʲiə.ɪjəxg] = godliness diadhaidheachd [dʲiə.ɛrʲ] = theologian diadhair [dʲiə.ɛrʲ] = theologian
Manx (Gaelg)	jee = god, deity, godhead ben jee = deity, goddess jeeoil = divine, godlike, godly jeeoilys = divinity, theology jeeoilagh = divine, theologian jeeaght = deity, divinity jeeaghteyr = theologian
Proto-Brythonic	*duɨw [ˈduɨ̯w] = god
Old Welsh	duiu = god
Middle Welsh (Kymreac)	duiu, duu, dyu, diu, duw = god dwyes, dwywes = goddess, noble lady, queen dywdap, dywdot, duwdab = the divine nature, divinity, theology dywol, duwiol = godly, devout, religious, pious, good, holy, sanctimonious
Welsh (Cymraeg)	duw / Duw [dɨu̯ / dɪu̯] = god, the Supreme Being, the Almighty, the Christian Trinity; O God! duwies, dwywes, dwyes = goddess, noble lady, queen duwdeb = the divine nature, divinity, theology duweiddiad = deification, apotheosis duweiddiaf, duweiddio = to deify, make a god of, turn into an object of worship duwgar = loving God, pious, devout duwiol, duwol = godly, devout, religious, pious, good, holy, sanctimonious
Old Cornish	duy = god
Middle Cornish (Cernewec)	dew, du, duy = god, God, a divinity dues = goddess
Cornish (Kernewek)	duw [dyˑʊ / diˑʊ] = god duwes = goddess duwonieth = theology duwonydh, duwonydhes = theologian Duw genes = goodbye (“god with you”)
Old Breton	doi = god
Middle Breton	doe, doué, dou = god doueadur = apotheosis, deification doueañ = to deify doueadur = apotheosis, deification doueañ = to deify doueegezh = divinity, deity doueek, doeel, doueel = divine
Breton (Brezhoneg)	doue [du] = god doueez =goddess douead = theist doueadegezh = deism doueegezh = divinity doueel = divine

Etymology: from Proto-Indo-European *deywós (god), from *dyew- (sky, heaven) [source]. The Deva rivers in Galicia get their name from the same Proto-Celtic root, and the Latin name for the city of Chester, Deva, possibly comes from the same Celtic root [source].

Words marked with a * are reconstructions.

Sources: Wiktionary, Etymological Dictionary Of Proto Celtic, In Dúil Bélrai English – Old Irish glossary, eDIL – Electronic Dictionary of the Irish Language, Teanglann.ie, Am Faclair Beag, Fockleyreen: Manx – English Dictionary, Online Manx Dictionary, Geiriadur Prifysgol Cymru, Lexicon cornu-britannicum : a dictionary of the ancient Celtic language of Cornwall, Gerlyver Kernewek, Devri : Le dictionnaire diachronique du breton, Dictionaire Favereau, TermOfis