banshee from Irish Gaelic bean sídhe "woman of the fairies"
bracket, breeches, brogue possibly from Gaulish brāca "trousers"
ceilidh from Irish Gaelic céilidh "social gathering"
coracle from Welsh corwgl
corgi from Welsh, from cor "dwarf" and ci "dog"
crag of Celtic origin, compare Irish/Scots Gaelic creag, Manx cregg, Welsh craig, altho the relationship between these Celtic words is obscure (OED)
cwm from Welsh cwm "valley"
Donald from Old Irish Domnall
druid from Latin druidae, from Proto-Celtic *dru-wid- "priest"
Fergus from Old Irish fer "man"
flummery from Welsh llymru "soft jelly from sour oatmeal"
galore from Irish Gaelic go leór "enough"
leprechaun from Irish Gaelic lupracán
MacKay from Scots Gaelic mac aoidh "son of the fiery one"
McIntyre from Irish Gaelic mac an tsaoir "son of the carpenter"
menhir from Breton maen-hir "long stone"
phoney from Irish Gaelic fáinne "ring"
ptarmigan from Scots Gaelic tàrmachan
qualtagh from Manx quaaltagh "the first person one meets after leaving the house"
slogan from Irish Gaelic sluagh-ghairm "battle-cry"
Tory from Old Irish tóir "pursue"
town from Proto-Germanic *tūnaz, thought to be borrowed from Proto-Celtic *dūnon "hill, stronghold"
whiskey from Irish Gaelic uisce beatha "water of life"