‘Anyone impugning the authority of St Jerome will be a heretic in the eyes of the western churches, whoever that man may be', wrote the Irish peregrinus Columbanus to Pope Gregory the Great. In this centenary year of Columbanus (died 615), I wish to explore which of the many facets of Jerome's life and work gave him such authority in the eyes of this Irishman. Columbanus certainly admired Jerome as a learned commentator and translator of the Bible. Beyond this, both men were deeply involved with propagating an extreme form of asceticism. But deeper even than this, I would argue, lies Columbanus's identification with Jerome as an ascetic who was critical of the clerical hierarchy within the church, from which he distanced himself. Given the excruciatingly short timeslots for communications, I will focus primarily on key letters of Jerome's which Columbanus knew; and I will explore their ideas on authority within the church and demonstrate their relevance for Columbanus's critical stance towards the Gallic bishops of his own day.