The conflicts that have deeply affected the island of Ireland in the twentieth century have been political, rather than religious, in basis. However, the powerful coalescence of Catholicism and nationalism on one hand, and Protestantism and unionism on the other, has meant that religious affiliation in Ireland has come to embody a wider range of cultural, political and social values. Furthermore, the successive waves of organised colonisation in the sixteenth and seventeenth centuries have left the island with the legacy of a distinctive religious geography, which became the explicit template for the political division of the island in 1921 as well as having profound implications for its development since the European Reformation of the early modern period. Yet, despite its centrality to Irish history and geography, it has been difficult to assess detailed change in that religious geography due to the inconsistency of territorial units over time. This article presents findings from a major research project funded by the UK’s Arts and Humanities Research Council which used Geographical Information Systems technology to standardise administrative boundaries across the island in the twentieth century. In doing so, it is able to shed new light on the radically divergent religio-spatial trajectories of different parts of the island in relation to political, social and economic developments.