While Old Irish (c. 600–900 A.D.) is extensively documented, it remains digitally under-resourced, lacking the range of digital resources available for other older Indo-European languages (e.g., Latin, see Pellegrini and Passarotti, 2018). We report on the development of a fully inflected lexicon of Old Irish nouns, provided in both phonemic and orthographic notation. This involved a computer-assisted, systematic, and reproducible grapheme-to-phoneme conversion pipeline and generating morphological forms through a finite-state transducer. The inflected lexicon we develop will better enable computational studies in Old Irish morphology, further research into diachronic developments, and have a wide range of Natural Language Processing (NLP) applications.