Words for harp, crwth and similar instruments, in Celtic languages:

Proto-Celtic	*krottos = round thing
Old Irish (Goídelc)	crott [krot] = harp, lute
Irish (Gaeilge)	cruit [kɾˠɪtʲ] = (small) harp; hunch, hump
Scottish Gaelic (Gàidhlig)	cruit [kruhdʲ] = (small) harp, lyre
Manx (Gaelg)	cruitçh = (small) harp, hump
Welsh (Cymraeg)	crwth [kruːθ] = crwth, crowd, fiddle, violin, viol; purring (of a cat); hump, hunch-back(ed), rounded, bent, convex; anything of round or bulging shape

Words marked with a * are reconstructions.

Old Irish (Goídelc)	cláirsech = harp
Irish (Gaeilge)	cláirseach [ˈklˠɑːɾˠʃəx / ˈklˠæːɾˠʃa(h)] = harp
Scottish Gaelic (Gàidhlig)	clàrsach [klˠaːrˠsəx] = harp, clarsach
Manx (Gaelg)	claasagh = harp

Etymology: possibly from the Proto-Celtic *klāros / *klārom (table)

Irish (Gaeilge)	teillén = swarm of bees
Welsh (Cymraeg)	telyn [ˈtɛlɨn / ˈteːlɪn] = harp
Cornish (Kernewek)	telyn = harp
Breton (Brezhoneg)	telenn = harp

Etymology: unknown. The word telyn first appeared in writing in a 17th century Cornish text. The Irish word teillén may or may not be related.

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

The crwth is also known as a crowd, cruth, crowth, crouth or rote in English. It is a kind of bowed lyre. It is particularly associated with Wales, but similar instruments were played in many parts of Europe from about the 11th century. It went out of fashion in the 18th century, but was revived in the 20th century. More information.