while anglo-saxon (and prolly other germanic groups’) mating patterns were shaken up starting as early the 600s, but definitely by the 800-900s, the irish carried on marrying their close relatives (close cousins and even uncle-nieces) until sometime well after the arrival of the normans in the twelfth century [pgs. 44-45 - links added by me]:
“But as far as the family and marriage were concerned, the wishes of the Church did not always prevail and resistance was often prolonged. The difference between ‘local custom’ and ecclesiastical law is nowhere stronger than in Ireland, even as late as the Norman period. It was then, during the late eleventh and early twelfth centuries, that English (or Norman) influence came to play a dominant role in reshaping the Irish Church. The archbishops of Canterbury, Lanfranc and Anselm, both protested against the Irish customs of marriage and divorce. From the seventh century Irish Church legislators had recognised only four degrees of kinship within which marriage was prohibited (and the law tracts fewer), whereas the papacy acknowledged seven…. ‘Native law’, comments Hughes (1966: 260), ‘triumphed over the stricter provisions of the church, to the disgust of the Anglo-Norman prelates, who were used to very different customs.’
“In theory, this state of affairs was altered by the first of the reforming synods, held at Cashel in 1101. However this conclave did not introduce the full requirements of the Roman Church, and, although it did forbid a man to marry his step-mother (or step-grandmother), or his sister or daughter, his brother’s wife, or any woman of similarly near kinship, it said nothing of the ‘Irish practices of concubinage and divorce.’ Even so, the legislation seems to have had little effect on social life, for some time later Pope Alexander III [pope from 1159 to 1181] was told that the Irish ‘marry their step-mothers and are not ashamed to have children by them; that a man may live with his brother’s wife while the brother is still alive; and that one many may live in concubinage with two sisters; and that many of them, putting away the mother, will marry the daughters’ (Sheehy 1962: I,21; Hughes 1966: 265).”
from the synod of whitby in 664 onwards, the anglo-saxons generally followed christian practices as they emanated from rome. the irish continued on with their own particular form of insular christianity until some 500 years later (same goes for some of the welsh, btw), and this affected their mating practices.
see also: The Development of the Family and Marriage in Europe by jack goody.
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