Seventeen-marker simple tandem repeat genetic analysis of Irish Y chromosomes reveals a previously unnoted modal haplotype that peaks in frequency in the northwestern part of the island. It shows a significant association with surnames purported to have descended from the most important and enduring dynasty of early medieval Ireland, the Ui Neill. This suggests that such phylogenetic predominance is a biological record of past hegemony and supports the veracity of semimythological early genealogies. The fact that about one in five males sampled in northwestern Ireland is likely a patrilineal descendent of a single early medieval ancestor is a powerful illustration of the potential link between prolificacy and power and of how Y-chromosome phylogeography can be influenced by social selection.