Words for fat, lard and related things in Celtic languages.

Old Irish (Goídelc)	blonac = fat, lard
Middle Irish (Gaoidhealg)	blonac, blonoc = fat, lard, grease
Irish (Gaeilge)	blonag = soft fat, lard, blubber, paunch, spare tyre blonag an mhíl mhóir = (whale) blubber blonagach = fat, greasy, flabby blonagán = goose-foot
Scottish Gaelic (Gàidhlig)	blonag [bl̪ˠɔnag] = visceral fat, suet, lard, tallow blonag-mhuice = lard
Manx (Gaelg)	blonnag = lard blennick = fat, fatty tissue, lard, blubber blennick vuickey = lard
Middle Welsh (Kymraec)	blonec, blonnec = fat, lard, grease
Welsh (Cymraeg)	bloneg = fat, lard, grease; belly, abdomen, lap, body fat; sap magu bloneg = to become fat meinwe floneg = adipose tissue blonegaf, blonegu, blonega = to grow fat, to make fat, to grease with far or lard blonegaidd = fatty, lardaceous, greasy
Old Cornish	blonec = fat
Middle Cornish (Cernewec)	blonec = fat, lard, grease
Cornish (Kernewek)	blonek = fat, lard, grease blonegek = greasy
Old Breton	blonecou = lard, fat
Middle Breton (Brezonec)	bloneg, blouhec, blounhec, blonec, bloanec = pork belly, fat blonegenn = fat bread, lard bread blonegenn-mor = jellyfish
Breton (Brezhoneg)	bloneg = addomen, lard, fat bloneg-mor = jellyfish blonegenn = fat bread, lard bread

Words marked with a * are reconstructions.

Etymology: unknown

Proto-Celtic	*gʷeress/*gʷered- = animal fat, tallow
Old Irish (Goídelc)	geir = fat, lard, suet, tallow fogeir = to heat, warm, inflame, chafe, irritate, temper
Middle Irish (Gaoidhealg)	geir = animal fat, tallow, suet, lard fo-geir = to heat, inflame, chafe, irritate, excite, temper
Irish (Gaeilge)	geir [ɟɛɾʲ] = fat, suet, tallow geir rósta = dripping geireach = fatty, sebaceous
Scottish Gaelic (Gàidhlig)	geir [gʲerʲ] = fat, lard, tallow geir-mhuc = lard geireach = fatty, greasy, full of suet geireadh = anointing, smearing with grease
Manx (Gaelg)	geirr = dripping, fat, suet, tallow geirr vill = beeswax
Middle Welsh (Kymraec)	guer, gwer = animal fat, tallow, suet, grease
Welsh (Cymraeg)	gwêr = animal fat, tallow, suet, grease gwêr (yr) aren = suet gweraidd = tallowy, tallowish gwerennaf, gwerennu = to congeal or coagulate into suet fat gwerennog = fat, suety, lardy, greasy

Words marked with a * are reconstructions.

Etymology: from Proto-Indo-European *gʷʰer- (heat) or *ǵʰwer- (wild animal) [source].

Words from the same PIE roots include furnace, gore, thermal and warm in English [source], gor (to hatch, incubate, warm, heat, burn) in Irish [source]., and words for warmth and heat in Celtic languages.

Old Irish (Goídelc)	sall, saill = bacon, salt-meat sallid = to salt, cure
Middle Irish (Gaoidhealg)	saill = salted meat (esp. bacon)
Irish (Gaeilge)	saill = salted meat, fat meat, fat; to salt, cure, season sailleach = fatty, adipose sailleadh = salting, curing sailleacht = saltiness (of meat, etc.) saillteoir = salter, curer
Scottish Gaelic (Gàidhlig)	saill [sail̪ʲ/sal̪ʲə] = to pickle, season; blubber, fat, suet saill-shùighte = saturated fat saill neo-shùighte = unsaturated fat saill nan dubhagan = suet saill bèicearachd = shortening saill muice-mara = blubber sailleadair = salter, fish-curer sailleadh = (act of) salting saillte = salted, salty saillear = (salt-)cellar
Manx (Gaelg)	sahll = adioise tissue, blubber, fat, grease sahllagh = adipose, blubbery, fatty

Words marked with a * are reconstructions.

Etymology: possibly from the Proto-Celtic *salanos (salt) , from Proto-Indo-European *séh₂ls (salt) [source]. Words for salt in Celtic and many other Indo-European languages come from the same roots [source].

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Gerlyvyr Cernewec, Dictionaire Favereau, TermOfis, Le dictionnaire diachronique du breton, Geriafurch, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic