Old Irish

Old Irish, also called Old Gaelic (endonym: Goídelc; Irish: Sean-Ghaeilge; Scottish Gaelic: Seann-Ghàidhlig; Manx: Shenn Yernish or Shenn Ghaelg), is the oldest form of the Goidelic/Gaelic language for which there are extensive written texts. It was used from c. 600 to c. 900. The main contemporary texts are dated c. 700–850; by 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 written at an earlier time. Old Irish is forebear to 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. Apparently, neither characteristic was present in the preceding Primitive Irish period, though initial mutations likely existed in a non-grammaticalised form in the prehistoric era.