Patrick’s purgatory in Lough Derg, Donegal, is one of the great medieval pilgrimage sites. On the extreme edge of Europe, it physically embodied the theological idea of purgatory, offering pilgrims a this-worldly encounter with its horrors. Despite Protestant efforts to destroy it, it survived the Reformation and posed a classic challenge for Protestant and Roman Catholic historians. For the latter, it exemplified the continuity of Catholicism in Ireland from Patrick to the present, a living embodiment of the unchanging loyalty of the Irish people to their national saint. For Protestant historians, it was, like purgatory, a twelfth-century invention, revealing both the medieval corruption of the Roman Catholic Church as it added non-scriptural embellishments to the Christian faith, and the superstition of Irish Catholicism. The interaction, and tension, between theological belief and historical objectivity was to prove a persistent challenge for both sides, right down to the twentieth century.