During and in the decades after the Irish Famine of 1846 to 1851, the population of the Ards Peninsula, Co. Down, both declined in numbers and changed in its composition. In 1863 the surnames of all resident householders were collected as part of a national valuation. These data have been used to calculate random isonymy values between civil parishes and the resultant matrix plotted by nonmetric multidimensional scaling. The results revealed both a major geographical barrier to gene flow, and politico-religious boundaries in existence since the early 17th century. The inter-relationships between random isonymy, geographical distance, and religious denominational frequencies within the peninsula were investigated by multiple regression analysis. While geographical influences predominated over short and long distances, in the intermediate distance range religion played the dominant role. The net effect of these subdivisions, in combination with famine-associated population losses, would have been to reduce local effective population sizes significantly, thus enhancing the potential for genetic drift and random inbreeding.

No abstract