De origine scoticae linguae (DOSL, also known as ‘O’Mulconry’s Glossary’) is an etymological glossary dating from around the late-seventh or early-eighth century. It discusses the origins of about 884 Irish words, very often deriving them from Latin, Greek or Hebrew. As such it represents the earliest etymological study of any European vernacular language. Despite this, however, the text has to date been almost completely ignored for its significance in the history of linguistics. This article analyses the authors’ methods, particularly with regard to the semantic and formal components of etymologies, and argues that the text shows considerable coherence, both internally and in relation to its sources and models in the Graeco-Roman linguistic tradition. It argues that DOSL is a serious work of scholarship that represents a milestone in the historical development of comparative linguistics.