The history of the Irish language begins with the period from the arrival of speakers of Celtic languages in Ireland to Ireland’s earliest known form of Irish, Primitive Irish, which is found in Ogham inscriptions dating from the 3rd or 4th century AD. After the conversion to Christianity in the 5th century, Old Irish begins to appear as glosses and other marginalia in manuscripts written in Latin, beginning in the 6th century. It evolved in the 10th century to Middle Irish. Early Modern Irish represented a transition between Middle and Modern Irish. Its literary form, Classical Gaelic, was used by writers in both Ireland and Scotland until the 18th century, in the course of which slowly but surely writers began writing in the vernacular dialects, Ulster Irish, Connacht Irish, Munster Irish and Scottish Gaelic. As the number of hereditary poets and scribes dwindled under British rule in the early 19th century, Irish became a mostly spoken tongue with little written literature appearing in the language until the Gaelic Revival of the late 19th century. The number of speakers was also declining in this period with monoglot and bilingual speakers of Irish increasingly adopting only English: while Irish never died out, by the time of the Revival it was largely confined to the less Anglicised regions of the island, which were often also the more rural and remote areas. In the 20th and 21st centuries, Irish has continued to survive in Gaeltacht regions and among a minority in other regions. It has once again come to be considered an important part of the island’s culture and heritage, with efforts being made to preserve and promote it.
Indo-European languages may have arrived in Ireland between 2,400 BC and 2,000 BC with the spread of the Bell Beaker culture when around 90% of the contemporary Neolithic population was replaced by lineages related to the Yamnaya culture from the Pontic steppe. The Beaker culture has been suggested as a candidate for an early Indo-European culture, specifically, as ancestral to proto-Celtic.Mallory proposed in 2013 that the Beaker culture was associated with a European branch of Indo-European dialects, termed “North-west Indo-European”, ancestral to not only Celtic but also Italic, Germanic, and Balto-Slavic.
The earliest written form of the Irish language is known to linguists as Primitive Irish. Primitive Irish is known only from fragments, mostly personal names, inscribed on stone in the Ogham alphabet. The earliest of such inscriptions probably date from the 3rd or 4th century. Ogham inscriptions are found primarily in the south of Ireland as well as in Wales, Devon and Cornwall, where it was brought by settlers from Ireland to sub-Roman Britain, and in the Isle of Man.