History of the Irish language
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.
Source: Wikipedia — History of the Irish language (CC BY-SA 4.0)