This research examines the significance of the Irish Boundary Commission 'episode' (July 1924 to December 1925) for Northern Ireland's nationalists. It tests the thesis that the inter-governmental agreement following the Commission's collapse in late 1925 - the agreement which cemented the 1920 border - was 'the key foundational moment' for the northern minority between December 1925 and 1939. Some writers view the episode as an important development in a larger story about partition generally: others recognise its deep significance for northern nationalists. But the literature says little about the construction and development of (often competing) nationalist narratives which flowed from it. There is no in-depth analysis of how it came to be remembered and shaped mind sets. The research interrogates, integrates and deploys archival material to produce a finer-grained reading of the period; and shows how arrangement of memories underpins narrative development. The dissertation considers how narratives about 1924/25 subsequently featured in political and popular discourse. In explaining the political culture which developed, it explores the relationships between the principal nationalist influences in the north and the two main political parties in the Irish Free State. The later chapters devote particular attention to the increasingly troubled relationship between border nationalists (especially in Fermanagh and Tyrone) and the• early Fianna Fail governments, led by de Valera. The research comments on clerical influence and engagement; and underscores the strength of the press in reinforcing cultural messages and values, and in fostering a sense of community. It offers an interpretation of why and how narratives about late 1925 so powerfully shaped the northern minority's attitudes and responses in later years. The dissertation concludes that, for northern nationalists, the episode, culminating in the December 1925 pact, had remained the organising principle in their political culture.