Studies of the theories of ethnicity, ethnology and ethnogenesis prevailing in early modern Ireland are noticeable by their absence. This dearth persists despite the fact that major periods of Irish history are understood as confrontations between differing, even incompatible, cultures or as conflicts between ethnic identities expressed as political ideologies – the Tudor expansion of the English state, the Cromwellian conquest, the Williamite War and even the Northern Irish Troubles are all interpreted in this manner. As there are few extant accounts of these theories, and their intimate connection to British and Irish political reality and practise, my research attempts to address this scholarly lacuna by charting the development of such ethnologies from the late Elizabethan period to the middle of the eighteenth century. Building on the work of Joep Leerssen and Clare O’Halloran, this research will look at the aspects of this topic that are still incomplete. Charles O’Conor of Belanagare is a significant presence in their accounts and, in the context of this thesis, his antiquarian works and historiography will be used to analyse preceding and prevailing ethnographical theories and ethnological attitudes. It is intended to first contextualize the nature of this ethnology by looking at those theories prevalent from 1690 to 1766 and to underscore their politically charged nature by analysing how they were shaped by political developments. Chapter one commences with a brief reference to the Lucas affair of 1748-50 and the fourth chapter returns to this controversy, exploring it in greater depth in order to analyse the nearest ethnographical dispute prior to the publication of O’Conor’s Dissertations on the Antient History of Ireland (1753). This work, alongside a history of the scholarly rehabilitations of the theory of Milesian ethnogenesis, is the subject of the fifth chapter. The second and third chapters deal with the gradual alterations in attitudes to various ethnological theories in Ireland between 1690 and 1750. The last chapter deals with O’Conor’s historiography, explaining the ethno-political theories adumbrated in his work, their uncontroversial reception among Irish Protestants, his activism after its publication and his attempt to contribute to Hiberno-Scottish historiographical debates.