The concepts of radicalisation and de-radicalisation are primarily defined by the assumption they make that there is a causal relationship between ideas and action. However, the causal role of ideas in informing behaviour has been strongly contested and has thus far eluded and undermined radicalisation and de-radicalisation conceptually and practically. The following article provides a theoretical basis for identifying the causal relationship between ideas and action through Margaret Archer’s critical realist ontology. Drawing on fieldwork conducted in Northern Ireland, the article identifies processes of ideational causal reproduction and morphogenesis in the Provisional Irish Republican Army’s thinking on armed struggle during its transition away from armed violence. It argues that the adoption of the Armalite and Ballot Box strategy in the 1980s introduced a contradiction in the movement’s ideology and that the movement was pressured to address this contradiction through three corrective cycles throughout the peace-process, subsequently softening its position on armed struggle.