The research presented here uses a novel method to show that contemporary party systems may originate much further back than is usually assumed or might be expected-in reality many centuries. Using data on Ireland, a country with a political system that poses significant challenges to the universality of many political science theories, by identifying the ancestry of current party elites we find ethnic bases for the Irish party system arising from population movements that took place from the 12th century. Extensive Irish genealogical knowledge allows us to use surnames as a proxy for ethnic origin. Recent genetic analyses of Irish surnames corroborate Irish genealogical information. The results are particularly compelling given that Ireland is an extremely homogeneous society and therefore provides a tough case for our approach. © 2011 The Authors. British Journal of Politics and International Relations © 2011 Political Studies Association.