In the nineteenth and twentieth centuries Catholicism was a defining element of Irish national identity and the terms Catholic and Irish were virtually synonymous for three-quarters of the population. The identification of faith and nationalist political identity, first harnessed by Daniel O’Connell, strengthened during successive efforts to end the Union and emerged triumphant in independent Ireland. Partition reinforced the association of political allegiance and religious affiliation on both sides of the border after 1920. Under the leadership of ultramontanist Cardinal Paul Cullen from 1852 until 1878, the Irish Catholic Church assumed a form that it maintained, remarkably, for over a century. This was characterised by a strong allegiance to Rome; a vast institutional presence through control of Catholic education, health and welfare homes; a disciplined clergy under episcopal control; and a thriving ‘spiritual empire’ abroad. The exceptional popular piety of twentieth-century Ireland sprang from Cullen’s success in standardising Irish devotional and liturgical life along Roman lines. The Cullenite model also fostered a pervasive culture of clericalism that lies at the heart of the unprecedented crisis in which the Catholic Church is now mired. This championed a clerical elite, institutional loyalty, conformity, anti-intellectualism and resistance to change. The power of religion peaked in the 1950s by which time the Catholic Church had become a lazy monopoly, the legacy of which is proving to be its greatest burden. The decline in the authority and pre-eminent position of the Catholic Church, the rise of secularism and the beginnings of the effort to dismantle legislative and constitutional support for a Catholic ethos can be traced to the early 1960s. Although identification with Catholicism and religious practice in Ireland remained atypically high and set Ireland apart from other parts of Western Europe, survey evidence since the 1970s has revealed dramatic change in the nature and practice of being Catholic. As elsewhere, scandal has engulfed the Catholic Church in Ireland since the 1990s. Yet despite the dramatic failure of leadership and the loss of power, credibility and moral authority of the institutional church, 84.2 per cent of Irish people described themselves as Catholic in the 2011 census. This suggests that despite a steep decline in institutional observance Catholicism remains an integral, if increasingly elusive, aspect of Irish identity.