It goes virtually without saying that the arrival in Ireland in the late 1160s of the first smallish contingents of Anglo-Norman adventurers and mercenaries marks a turning-point of some sort in the history of Ireland's relationship with Wales. Frequently the invaders had cut their teeth in Wales, many had won estates in Wales or in the Welsh marches and some had found wives there. They brought with them to Ireland, and employed in their wars against the Irish, valuable experience gained in comparable warfare against the Welsh. They countered Irish arms with bands of Welsh archers and colonized their newly acquired swordland with ship-loads of tenants from their Welsh and Marcher lordships, some of whom were undoubtedly of native Welsh extraction.

The invasion established, therefore, a new nexus of involvement between Ireland and Wales, which interrupted, though it certainly did not obliterate, earlier patterns of contact between the two countries. This operated at different levels – ecclesiastical and scholarly, political and military, commercial – and also, and equally significantly, with different degrees of intensity. For instance, if one were to draw a line on a map of Ireland running from Galway to Dundalk, the evidence would indicate that the area north of that line, as geography if nothing else dictated, had long-established lines of communication with northern Britain and was by comparison relatively isolated from contact with Wales.