This study offers an account of the Irish church during the twelfth century, a time of institutional restructuring and religious renewal associated with a reform movement that was a regional manifestation of a much wider European phenomenon. The sources for such an undertaking are problematic, with serious gaps in the evidence, making a comprehensive portrayal difficult to achieve for a time when elsewhere in medieval Europe there was a widespread increase in the quantity and quality of written sources. Although the focus is the pre-Anglo-Norman twelfth century it has sometimes proved necessary, in light of evidentiary problems, to extend the chronological parameters back before the twelfth century, or forward into the post-Anglo-Norman period. Anglo-Norman intrusion into Ireland from 1167 onwards undoubtedly had a major impact on the Irish church, but it merits separate treatment and is not attempted here. Nonetheless, it has cast a retrospective shadow over the twelfth-century Irish church as a moral imperative for external intervention was advanced on the basis of an urgent need for reform. The decadence and the general backwardness, even barbarity, of Irish society, as well as its imputed nominal practice of Christianity, were used to justify Anglo-Norman engagement with Ireland, most notably in the propagandist writings of its principal apologist, Gerald of Wales. Gerald's case relied on drawing a deliberately sharp contrast between natives and newcomers, leading to exaggerated portraits of Irish degeneracy.