Words for dwelling, settlement, town, tribe and related things in Celtic languages.

Proto-Celtic	*trebā = dwelling
Gaulish	Atrebates = name of a tribe
Old Irish (Goídelc)	treb = house, farm; household; tribe
Middle Irish (Gaoidhealg)	treb = house, farm; household; tribe
Irish (Gaeilge)	treibh [ˈtʲɾʲɛv] = house, homestead, farmstead; household, family; tribe, race
Scottish Gaelic (Gàidhlig)	treabh [tro] = farming village treubh [treːv] = tribe
Proto-Brythonic	*treβ [ˈtrɛːβ] = town, settlement
Middle Welsh (Kymraec)	tref [ˈtrɛːβ] = town, settlement trefad, trevad, treuad, treuat = dwelling(-place), habitation trefan, treuan = small town or city, village, dwelling, abode, manor-house, fort
Welsh (Cymraeg)	tre(f) [treː(v)] = town; town centre; dwelling(-place), habitation, residence, home; house (and surrounding land), homestead, farm, estate, cluster of houses; township; tribe trefad = dwelling(-place), habitation, residence, home, region, domain trefaf, trefu= to dwell, live, settle trefan = small town or city, village, dwelling, abode, manor-house, fort
Middle Cornish (Cernewec)	tre, trev = dwelling place, homestead, home, town tregva, trigva = dwelling place, habitation trevedic = one from a country village, a country man, rustic
Cornish (Kernewek)	tre [trɛ:/tre:] = farmstead, home, town, village trigva = abode, address, dwelling, resdidence trigys = settled, to reside, live
Old Breton	treff, treb = town, settlement
Middle Breton (Brezonec)	tre, trev = town
Breton (Brezhoneg)	trev = town trevour = civil

Etymology: From the Proto-Indo-European *treb- (dwelling, settlement) [source].

Words from the same Proto-Celtic root (via Latin) possibly include trobo (beehive, skep) in Galician, and truébanu (beehive, barrel, basket) in Asturian [source].

This is also the root of the archaic English word thorp(e) (a group of houses standing together in the country; a hamlet; a village), which appears in place names such as Milnthorpe and Scunthorpe.

Related words in other languages include Dorf (hamlet, village, town) in German, torp (village) in Danish, torp (farm, cottage, croft) in Swedish, þorp (village, farm) in Icelandic, and trevë (country, region, village) in Albanian [source].

Words marked with a * are reconstructions.

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