APPROVED This thesis collects, catalogues, and analyses synchronic language variation in Old Irish sources in order to investigate the possibility of Old Irish dialects. This thesis represents the first comprehensive attempt to provide a systematic overview and analysis of attested variant forms and their possible explanations, by building upon previous research discussing linguistic variation in the Old Irish corpus through the collation of prior suggestions and original findings into a cohesive collection. The primary methodology employed in this thesis is that of collection and analysis: assembling a list of variant features and collecting examples of their occurrence from three corpora of Old Irish material, selected based on their chronological closeness and similarity of content. Variant features were identified from a thorough review of existing literature as well as original collections from the Dictionary of the Irish Language and from the material itself, during the course of the study. The thesis dismantles the assumption that Old Irish is a language free from any variation and instead proposes understanding Old Irish as a scholastic written standard, and deviations from that standard to be viewed as slips into a different stratum of language, whether it be diatopic or diastratic. It thoroughly examines identified potential variants and discusses their functions and potential origins. This study has been successful in its aim of identifying synchronic language variation in Old Irish and, indeed, has identified a potential diatopic variant that may be used in future scholarship to further narrow down the geographic origins of a particular manuscript.