This term, which describes the Christian church as it developed in Wales, Ireland, and Scotland, recognizes that church practice in all three countries had many features in common, but should not disguise the fact that there were very real differences between them. In particular, the concept of the territorial episcopal diocese was based on the administrative divisions of the Roman empire, with which Ireland had no formal link, and consequently the diocesan system had difficulty in taking root there. At the time of the Roman withdrawal much of Wales and Scotland was still heathen, and the earliest exact date for the presence of Christians in Ireland is a reference in 431 to Palladius, bishop to ‘the Irish who believe in Christ’. Ireland was evangelized largely from Britain, its most famous British missionary being St Patrick, in the early and mid‐5th cent. The diffusion of the cult of Patrick, and the growth in the status of Armagh, the ecclesiastical centre most closely associated with him, parallels that of St David in Wales, while the arrival of the Irish saint Columba in Iona (Scotland), in 563, marked the start of a lengthy period of Irish missionary activity in Britain and the continent.