This PhD dissertation explores how rivalry, understood as a protracted intractable conflict, emerges, and develops. It takes issue with current approaches to rivalry in International Relations (IR) and Peace and Conflict Studies (P&CS) to advance an alternative, innovative, and interdisciplinary Mimetic Rivalry Framework (MRF) that draws on René Girard’s Mimetic Theory. In contrast to existing research on rivalry in IR and P&CS, the MRF argues for the distinctive nature of rivalry vis-àvis other forms of conflict. It challenges several common assumptions – such as the rationalist assumption on human agency, the materialist assumption on rivalry emergence, and the reification of identity as a difference marker – while embracing complexity, relationality, and mimeticism. Informed by several multi-disciplinary theories – namely, Clyde Kluckhohn’s Social Value Theory, Gabriel Tarde’s Theory of Value, and Leon Festinger’s Social-Comparison Theory – the MRF offers a set of indicators able to empirically trace the stages leading an inter-group dyad from a state of non-competition to the stage of rivalry emergence and, finally, to a full-fledged rivalry. To test the empirical robustness of the framework, the thesis then investigates the rivalry between Republicans/Nationalists/Catholics and Loyalists/Unionists/Protestants in Northern Ireland as it developed around two intractable issues: that of territory (1964-1998), and that of language (1998-2020). In contrast to the existing literature, the MRF shows that the atypical intractability of the Northern Ireland rivalry is a product of the extreme similarity of desires, rather than of their difference. Because intractable difference is an artefact of mimetic rivalry, this dissertation argues that the politics in Northern Ireland remains prone to rivalry’s chronic intractability unless the underlying mimetic matrix of mutually exclusive identities is unveiled and challenged. Finally, the thesis suggests that recent developments in communal relations in the Irish language community indicate that the Irish language has the potential for enabling communal reconciliation.