This thesis examines the causal pathways underpinning the moderation of radical ethno-nationalism using the case of Irish republicanism (Sinn Féin and the IRA) between 1969 and 2010. Through the application of the ‘inclusion-moderation’ framework, I argue that a strong macro-institutional framework is central to the process of moderation. Existing explanations that emphasise the role of interplay, exchange and leadership choices typically neglect the importance of this wider institutional framework in enabling and shaping the decisions made. In the case of Irish republicanism, the processes of electoral participation, bargaining to design stable democratic institutions, and securing credible guarantees to protect their interests from the United States, all combined and reinforced each other to create a scenario whereby republicans moderated. These processes hinged upon stable democratic institutions that were perceived by republicans as embodying relatively low risks for participation, providing a stable basis for future competition, and rendering the future of Northern Ireland as part of the United Kingdom potentially uncertain. Moderation was a gradual and path-dependent process of increasing returns whereby contact with the stable institutions imposed constraints against radicalism and incentives towards moderation. Republicanism’s transformation is best understood as moving through a series of phases, beginning with absolute radicalism, moving to relative radicalism, before becoming moderate. Crucial to this process was the decision to participate in institutions, which changed and regulated their relationships with other actors, requiring them to build alliances with potential supporters and political opponents. However, moderation was a layered process with some aspects of their policies and beliefs becoming moderate while others remained radical, albeit over time their remaining radicalism became completely accommodating. This was about acquiescing to a system of political order rather than core value change. Republicans continue to assert an alternative claim to sovereignty, reject the legitimacy of British ruling institutions, and continue to assert the legitimacy of their right to armed struggle, albeit they have put the use of violence in their past. As such, rather than thinking of ethno-national radicalism as entailing value change to prove the sincerity of their moderation, it is preferable to look to the ways they demonstrated a commitment to their new moderate path, such as through the process of decommissioning, their endorsement of policing in Northern Ireland, and their response to ongoing threats of violence from former dissident comrades. In short, the inclusion-moderation theory is a powerful approach for explaining ethno-national moderation but it needs some modification for the ethno-national context.