Insular Celtic languages are the group of Celtic languages spoken in Brittany, Great Britain, Ireland, and the Isle of Man. All surviving Celtic languages are in the Insular group, including Breton, which is spoken on continental Europe in Brittany, France. The Continental Celtic languages, although once quite widely spoken in mainland Europe and in Anatolia, are extinct.
Six Insular Celtic languages are extant (in all cases written and spoken) in two distinct groups:
Brittonic (or Brythonic) languages: Breton, Cornish, and Welsh
Goidelic languages: Irish, Manx, and Scottish Gaelic
The "Insular Celtic hypothesis" is a theory that they evolved together in those places, having a later common ancestor than any of the Continental Celtic languages such as Celtiberian, Gaulish, Galatian, and Lepontic, among others, all of which are long extinct. This linguistic division of Celtic languages into Insular and Continental contrasts with the P/Q Celtic hypothesis.
The proponents of the Insular hypothesis (such as Cowgill 1975; McCone 1991, 1992; and Schrijver 1995) point to shared innovations among these – chiefly:
inflected prepositions
shared use of certain verbal particles
VSO word order
differentiation of absolute and conjunct verb endings as found extensively in Old Irish and less so in Middle Welsh (see Morphology of the Proto-Celtic language).
The proponents assert that a strong partition between the Brittonic languages with Gaulish (P-Celtic) on one side and the Goidelic languages with Celtiberian (Q-Celtic) on the other, may be superficial, owing to a language contact phenomenon. They add the identical sound shift (/kw/ to /p/) could have occurred independently in the predecessors of Gaulish and Brittonic, or have spread through language contact between those two groups. Further, the Italic languages had a similar divergence between Latino-Faliscan, which kept /kw/, and Osco-Umbrian, which changed it to /p/. Some historians, such as George Buchanan in the 16th century, had suggested the Brythonic or P-Celtic language was a descendant of the Picts' language.