Old Irish, attested from roughly the sixth through tenth centuries CE, is the earliest Celtic language with a substantial surviving vernacular literature, belonging to the Goidelic branch of the Celtic family within Indo-European, and Irish monasteries, centers of considerable learning during the early medieval period often called the Age of Saints and Scholars, produced not only extensive Latin Christian scholarship but also a remarkably early and substantial body of vernacular Old Irish literature, unusual for medieval Europe, where Latin otherwise dominated written culture for centuries longer in most regions.
Old Irish preserves substantial mythological and legendary material concerning pre-Christian Irish religious and heroic tradition, most significantly through the Ulster Cycle, centered on the hero Cu Chulainn and the epic Tain Bo Cuailnge, and through texts describing the Tuatha De Danann, a race of divine or semi-divine beings associated with pre-Christian Irish religion, whose stories were recorded by Christian monastic scribes working centuries after Ireland's conversion to Christianity in the fifth century. Old Irish grammar is notoriously complex, particularly its system of initial consonant mutation, in which a word's first sound changes depending on grammatical context, a feature shared across the wider Celtic language family and one that made the language a significant object of dedicated modern linguistic study once substantial critical grammars became available in the twentieth century.