I agree. See Uí Liatháin, Déisi, and Llŷn Peninsula. The connection is principally with southern Ireland and dates from Antiquity. Alliances are also known from the Irish sagas. Lugaid mac Con supposedly gained his throne thanks to one, while one was made against Conaire Mór. In fact there are whole speculated Irish population groups which some scholars believe originated in Britain, speaking languages closely akin to the Brittonic ancestor of Welsh.