The term Insular Celtic refers to those Celtic languages which originated in the British Isles, in contrast to the Continental Celtic languages of mainland Europe and Anatolia. All surviving Celtic languages are from the insular Celtic group. Continental Celtic languages are extinct. The six Celtic languages of modern times can be divided into:

• the Goidelic languages (Irish, Manx, and Scottish Gaelic); and
• the Brythonic languages (Breton, Cornish and Welsh).

Insular Celtic hypothesis

The "Insular Celtic hypothesis" is a theory that the Brythonic and Goidelic languages evolved together in those islands, having a common ancestor more recent than any shared with the Continental Celtic languages such as Celtiberian, Gaulish, Galatian and Lepontic, among others, all of which are long extinct.

The proponents of the Insular Celtic hypothesis (such as Cowgill 1975; McCone 1991, 1992; and Schrijver 1995) point to shared innovations among Insular Celtic languages, including inflected prepositions, shared use of certain verbal particles, VSO word order, and the differentiation of absolute and conjunct verb endings as found extensively in Old Irish and to a small extent in Middle Welsh (see Proto-Celtic language). They assert that a partition that lumps the Brythonic languages and Gaulish (P-Celtic) on one side and the Goidelic languages with Celtiberian (Q-Celtic) on the other may be a superficial one (i.e. owing to a language contact phenomenon), as the identical sound shift ( to ) could have occurred independently in the predecessors of Gaulish and Brythonic, or have spread through language contact between those two groups.

The family tree of the Insular Celtic languages is thus as follows:

• Insular Celtic
  • Goidelic
    • Primitive Irish, ancestral to:
      • Old Irish, ancestral to:
        • Middle Irish, ancestral to:
          • Irish
          • Scottish Gaelic
          • Manx
  • Brythonic
    • Pictish (possibly)
    • British
      • Cumbric (extinct)
      • Old Welsh, ancestral to
        • Middle Welsh, ancestral to:
          • Welsh
      • Southwestern Brythonic, ancestral to:
        • Breton
        • Cornish

The following table lists cognates showing the development of Proto-Celtic * to in Gaulish and the Brythonic languages but to in the Goidelic languages.

A significant difference between Goidelic and Brythonic languages is the transformation of *an, am to a denasalised vowel with lengthening, é, before an originally voiceless stop or fricative, cf. Old Irish éc "death", écath "fish hook", dét "tooth", cét "hundred" vs. Welsh angau, angad, dant, and cant. Otherwise:

• the nasal is retained before a vowel, jod, w, m, and a liquid:
  • Old Irish ban "woman" (< banom)
    • • Old Irish gainethar "he/she is born" (< gan-je-tor)
        • • Old Irish ainb "ignorant" (< anwiss)
            • the nasal passes to en before another n:
              • Old Irish benn "peak" (< banno) (vs. Welsh bann)
                • • Middle Irish ro-geinn "finds a place" (< ganne) (vs. Welsh gannaf)
                    • the nasal passes to in, im before a voiced stop
                      • Old Irish imb "butter" (vs. Breton aman(en)n, Cornish amanyn)
                      • Old Irish ingen "nail" (vs. Old Welsh eguin)
                      • Old Irish tengae "tongue" (vs. Welsh tafod)
                      • Old Irish ing "strait" (vs. Middle Welsh eh-ang "wide")

                    References

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