Unification
Saxon connections with the greater Latin West led to papal preferment and brought the Celtic-speaking peoples into closer contact with the orthodoxy of the councils. The customs and traditions particular to Insular Christianity became a matter of dispute, especially the matter of the proper calculation of Easter. Synods were held in Ireland, Gaul, and England (e.g. the Synod of Whitby) but a degree of variation continued in Britain after the Ionan church accepted the Roman date.
The Easter question was settled at various times in different places. The following dates are derived from Haddan and Stubbs: South Ireland, 626-8; North Ireland, 692; Northumbria (converted by Celtic missions), 664; East Devon and Somerset, the Celts under Wessex, 705; the Picts, 710; Iona, 716-8; Strathclyde, 721; North Wales, 768; South Wales, 777. Cornwall held out the longest of any, perhaps even, in parts, to the time of Bishop Aedwulf of Crediton (909).
A uniquely Irish penitential system was eventually adopted as a universal practice of the Church by the Fourth Lateran Council of 1215.
Read more about this topic: Celtic Christianity