Phonological history of Old Irish

Old Irish was affected by a series of phonological changes that radically altered its appearance compared with Proto-Celtic and older Celtic languages (such as Gaulish, which still had the appearance of typical early Indo-European languages such as Latin or Ancient Greek). The changes occurred at a fairly rapid pace between 350 and 550 CE. == Summary of changes == A capsule summary of the most important changes is (in approximate order): Syllable-final *n (from PIE *m, *n) assimilated to the following phoneme, even across word boundaries in the case of syntactically connected words.

Source: Wikipedia — Phonological history of Old Irish (CC BY-SA 4.0)