Words for to die in Celtic languages.

Proto-Celtic	*marwos = dead
Old Irish (Goídelc)	marb = dead; mortified, insensible, spiritually dead; stagnant (water)
Irish (Gaeilge)	marbh [ˈmˠɑɾˠəvˠ / ˈmˠarˠuː / ˈmˠarˠu] = dead person, dead, inert, exhausted, inactive, motionless, slack, idle, unused
Scottish Gaelic (Gàidhlig)	marbh [marav] = dead, lifeless; dull; benumbed, torpid; vapid, tasteless; to kill, murder, assassinate
Manx (Gaelg)	marroo = to kill, kill off, dispatch, murder, assassinate, slaughter, liquidate, butcher, slay, exterminate, bag (game); dead, lifeless, inanimate, glassy (look), muggy, dull (pain), dud, dull, stagnant, defunct, mortified, slaughtered, departed, deceased, killed, extinct
Proto-Brythonic	marw [ˈmarw] = to die
Welsh (Cymraeg)	marw = to die, expire, cease to exist, vanish, fade away
Cornish (Kernewek)	merwel [‘mɛrwɛl / ‘mɛrwɐl] = to die, decease, pass away, go out (light)
Middle Breton	marf / maru = to die
Breton (Brezhoneg)	mervel = to die, turn off, switch off, go out, become insensitive

Etymology: from the Proto-Indo-European *mer- (to die), which is also the root of the English words mare, as in nightmare, and murder [source].

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

Irish (Gaeilge)	básaigh = to die, put to death, execute
Scottish Gaelic (Gàidhlig)	bàsaich [baːsɪç] = to die, perish, starve, wither
Manx (Gaelg)	geddyn baase = to die, perish, decease

Etymology: from the Irish bás (death), from the Proto-Celtic *bāstom / bāssom (death), from Proto-Indo-European *gʷeh₂- (to go) [source].

Sources: Wiktionary, Am Faclair Beag, On-Line Manx Dictionary, Teaglann.ie