The phenomenon of ‘Ireland’s spiritual empire’, denoting the influence that Irish churches had on the world through lay and clerical migration in the (very) long nineteenth century, has attracted considerable attention from both contemporary commentators and historians. Yet the converse reality that national churches so embroiled in the global growth of their religions must also have undergone far-reaching changes themselves in the process has been much less studied. Focusing on both Catholic and Protestant churches, this chapter will address a number of modes of religious ‘Americanisation’ that can be detected in Ireland between 1841 and 1925. These will include: the backflow of a ‘cosmopolitan clergy’ who frequently spent long periods in North America and returned to Ireland as potential agents of a religiously inflected Americanisation; the visits of Irish-American and ‘Scotch-Irish’ clergy to Ireland; and the material role that a much-vaunted American religious ‘freedom’ played in the imaginaries of both Catholic and Protestant Irish people, enhanced by both media portrayals and discussion in personal correspondence.