Those who settled in Ireland and further conquered Scotland were representatives of the other, the Goidelic subgroup. Celts had invaded these islands in the 8th century B.C. and soon assimilated the aboriginal tribes, obviously non-Indoeuropean. Nevertheless, many think that pre-Celtic populations existed here much later than was at one time assumed and influenced Celtic tongues right up to the beginnings of the new, A.D. era, and that their influence caused the forming of initial mutations and other peculiarities of the Insular Celtic languages. These British people spoke a language which was not greatly different from Continental Celtic tongues, Gaulish and others. But while the language of the former had been evolving, changes did occur.
Diphthongs tend to disappear here, turning into simple vowels.
In the 6th century Angles, Saxons and Jutes invaded the island, and after a long and severe struggle Britons were pushed back into Wales and Cornwall, from which some of them left for Brittany, back to the continent. The language broke up into three separate groups, two of them remaining in existence at the present time.