A new edition of the earliest lexical study of a European vernacular language. De Origine Scoticae Linguae (also known as O'Mulconry’s Glossary) is a text originating in seventh-century Ireland that provides etymologies for c. 880 Irish words, mostly drawn from Latin, Greek, and Hebrew. Its Latin prologue declares its affiliation to the Graeco-Roman linguistic tradition, claiming an origin for the Irish language in the Greek dialects Attic, Doric and Aeolic. The glossary attests to the transmission and reception of the Latin grammatical tradition in Ireland and shines light in particular on the Irish knowledge of Greek and Hebrew. The text also represents a milestone in the history of European linguistics, as the earliest etymological study of a European vernacular language. The glossary was published once before, by Whitley Stokes in 1898. This new edition provides the first translation and textual commentary, clarifying the sense of difficult entries and discussing sources. The introduction analyses the structure and contents, origins and development, linguistic issues, and relationships to other texts. The text is edited here along with a shorter related glossary of 232 entries, entitled Irsan, which includes shared material and sheds further light on its development.