The Celtic languages are one of the most important chapters of European language, yet that chapter is at the precipice. Only six Celtic languages are spoken today with any native currency, all of them Insular Celtic languages, and no Celtic language can be described as "thriving." Manx and Cornish are virtually dead; Breton may die within our lifetimes; Welsh soon after our lifetimes; Scots Gaelic is in sharp decline; and Irish, also in decline, sits at the cliffedge, its fate still undecided.
Proto-Celtic was spoken around 2000-1000 BCE and quickly spread through migration (and, later, conquest) throughout central Europe and the British Isles. The sea between the Isles and the rest of Europe created a linguistic divide as well. Proto-Celtic soon divided into Continental Celtic and Insular Celtic. Continental Celtic broke into Gaulish, Celtiberian, Lepontic, among others. Insular Celtic of the British Isles became Goidelic and Common Brittonic. All the Celtic languages alive today are Insular Celtic tongues - even Breton, which came to France from Irish settlers in early history.
Proto-Celtic was not a language isolated from its neighbors. The language was spoken in Central-to-Midwestern Europe, associated with the Halstatt culture in pre-history. Celtic replaced the native languages in the region, and the tongue(s) of the land were lost. The language absorbed words from neighbors, especially trade words for metallurgy. Apart from obvious Wanderwörter, there is no clear line between loans from neighbors and loans from replaced languages. The only conservative conclusion is that there are substrata in Proto-Celtic that were formed from unknown sources.
*agr- "tree". For a full discussion, see *agr- in Pre-Proto-Germanic. Kroonen 2014.
*agr-on-a: "berries," "fruits." Proto-Celtic.
aeron "id." Welsh.
*agrn-io- "sloes," "berries," "plums" Proto-Celtic.
*-ana- "fruit". For a full discussion, see *-ana- in Pre-Proto-Germanic. Kroonen 2014.
*agr-on-a: "berries," "fruits." Substratum meaning was evidently agr- "tree" + -on-a: "fruit." Proto-Celtic.
aeron "id." Welsh.
BAS? "red," "purple". Northwest European. A PIE etymology is possible as *bhas- but challenged by the fact that there is extremely limited attestation and it is geographically confined to Northwest Europe. Beekes 1996; Matasovic 2008.
*basko- (adj.) "red". Proto-Celtic.
basc (o. m.) "red" Middle Irish. Appears only in glossaries.
basu (adj.) "purple". Old English.
beri "berry". Old High Germanic. Kluge disconnects this from the Celtic and Old English forms and derives this from PIE *h3gwh-os- with cognates in Balto-Slavic.
BHIKO? "bee". European Substratum. Geographic implications imply a loan from a Western European source. A PIE reading as *bhi-, although the Latin word implies *bhoy-. [The disagreeing suffixes *-ko- and *-te- point to an unknown PIE stem or suffixing pattern in a non-IE language]. Matasovic 2008.
bec Limousin French.
bech (o.m.) Old Irish.
be/ygegyr (m.) "drone". Middle Welsh.
be:o- Old English.
bьčela Old Church Slavonic.
ELM? See Pre-Proto-Germanic.
*gis-nó- "pine tree". See Pre-Proto-Germanic. Kroonen 2014.
GLEND? A PIE etymology as *glend- is possible but the geographic to limitation to Celtic and Germanic leaves open the possibility of a loan. Matasovic 2008.
*glendos- (n.) "valley," "shore". Proto-Celtic.
klint "shore". Middle Low Germanic.
klettr "rock". Old Norse.
KAG? PIE *kagh- is conceivable but highly unlikely. Matasovic 2008.
*kag-o- (v.) "gets," "receives." Proto-Celtic.
in-cohare (v.) "to begin." Latin.
kahad (v.) "takes." Oscan.
*klamo- (adj.) "sick," "leprous". The *a points to a non-IE source. [Link to English calamity ?]. Matasovic 2014.
clam (o.) "leprous". Old Irish.
claf "sick," "leprous". Middle Welsh.
claff Middle Breton.
claf ld Cornish.
MESAL? "blackbird". Northwest European. A PIE etymology as *mes-al- is phonologically possible but Beekes (1996) and Matasovic (2008) note that the word is geographically confined to Northwest Europe, making a source from the north of the Caucasus difficult.
amusla "blackbird" Old High Germanic.
amsala "blackbird" Old High Germanic.
*mesal-ka:- "blackbird". Proto-Celtic.
mwyalch (f.) "blackbird". Middle Welsh.
moualch (f.) Middle Breton.
moelh Old Cornish.
merula "blackbird". Latin.
MEN? (adj.) "many," "frequent". Phonologically possible as PIE *men- but the Slavic and Germanic forms point to a non-IE shape of the root. Boutkan & Siebinga 2005; Matasovic 2008.
manig Old English.
meinicc (i.) Old Irish.
mynych Middle Welsh.
mъnogъ Old Church Slavonic.
*molto- (n.) "ram". Matasovic 2014.
*nino- "ash tree." Pre-Proto-Celtic. Matasovic 2014.
*skublo- (n.) "predatory bird". Bird Language. Matasovic (2014) notes that in the absence of cognates, this was probably borrowed.
*subi- (n.) "strawberry". Matasovic (2014) notes that the presence of a neuter i-stem in Irish looks archaic.
*trusto- (n.) "noise". Matasovic 2014.
trost (o.m.) Middle Irish.
trwst Middle Welsh.
trous Middle Breton.
tros ld Cornish.
Boutkan, Siebinga. Old Frisian Etymological Dictionary. 2005.
Beekes, R. S. P. "Ancient European Loanwords". Historische Sprachforschung. Vol. 109. 1996.
Kroonen, Guus. Etymological Dictionary of Proto-Germanic. 2008.
Matasovic, Ranko. Etymological Dictionary of Proto-Celtic. 2008.