Wikipedia
Old Irish (sometimes called Old Gaelic) is the name given to the oldest form of the Goidelic languages for which extensive written texts are extant. It was used from c. AD 600–900. The primary contemporary texts are dated c. AD 700–850; by AD 900 the language had already transitioned into early Middle Irish. Some Old Irish texts date from the 10th century, although these are presumably copies of texts composed at an earlier time period. Old Irish is thus the ancestor of Modern Irish, Manx, and Scottish Gaelic.
Old Irish is known for having a particularly complex system of morphology and especially of allomorphy (more or less unpredictable variations in stems and suffixes in differing circumstances) as well as a complex sound system involving grammatically significant consonant mutations to the initial consonant of a word. Initial consonant mutation must have been present in at least late Common Celtic (Proto-Celtic) because this distinguishing feature has survived with grammatical significance in both modern Welsh and Breton, and the extinct Cornish language also featured. Because the languages belong to the Brittonic branch of the Celtic language group (so-called "P-Celtic"), initial mutation must predate the split in the development paths of the Brittonic and Goidelic languages. No mutations are, however, attested in Gaulish material so a parallel evolution of the phenomenon in the neo-Celtic languages is also possible. Much of the complex allomorphy has been lost, but the rich sound system has been maintained, with little change, in the modern languages.
Contemporary Old Irish scholarship is still greatly influenced by the works of a small number of scholars active in the late 19th and early 20th centuries such as Rudolf Thurneysen (1857–1940) and Osborn Bergin (1873–1950).