Old Irish

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.

Old Irish literature's preservation of pre-Christian Irish mythology, including the Tuatha De Danann and their eventual retreat underground to become the sidhe, fairy or otherworldly beings of later Irish folklore, occurred entirely through Christian monastic scribes, meaning surviving Old Irish mythological texts represent a Christianized reworking and euhemerization of earlier oral pagan tradition rather than an unmediated pagan source, a source-critical situation similar to that facing Old Norse mythology preserved through Snorri Sturluson's Christian-era Prose Edda.

The Book of Invasions, Lebor Gabala Erenn, compiled in Old and Middle Irish, presents a pseudo-historical account of successive legendary peoples settling Ireland, including the Tuatha De Danann, framed within a broader biblical chronological scheme that Christian compilers used to reconcile native Irish legendary history with scriptural world history. Ogham, an early medieval Irish alphabet consisting of notches carved along a central line, was used to write Old Irish, chiefly on stone memorial inscriptions, and though its precise origins and full range of uses remain debated among scholars, some later medieval sources associate it with druidic or magical significance beyond simple memorial inscription.