Quite the most remarkable achievement of nineteenth-century Ireland was the creation of an international Catholic Church throughout the Celtic diaspora in the British Empire and North America. A true Irish empire beyond the seas, it was often compared in Hibernian self-congratulation to the monastic missions of the Dark Ages and was served by an Irish clergy and a host of religious orders who fostered a distinctively ‘ethnic’ or Irish Catholic expatriate culture, while often showing the higher values of the Catholic spiritual life. It is remarkable that there is no scholarly modern study of this international community now in process of dissolution, for it has given an incalculable strength to twentieth-century Roman Catholicism. Something of its dimensions and importance can, however, be glimpsed from a growing body of historical writing about Irish Catholicism in England and Scotland, the United States and Australia, as well as in Ireland itself. The American Republic and the white settler areas of the British Empire were to Irish Catholics what the Roman Empire had been to Jews and Christians, the alien organisms by which a faith was carried to the far corners of the earth. As a matter of institutional and ecclesiastical history, the subject is one in which the new nations were divided into dioceses and parishes, and provided with churches, convents, colleges, seminaries and schools. This was, moreover, achieved by no easy process, but in spite of endemic conflict within Irish Catholic communities, who were also opposed by Roman Catholics of other national traditions, by the expanding Protestant Churches and by a hostile Protestant or secular state.