Despite well-known benefits of physical activity, in Ireland only 38 % of older adults are sufficiently active. Behavioural interventions are rarely developed systematically and, when reported, inadequate description often becomes a barrier for subsequent replication and scalability. In this article, we describe the development and characteristics of Move for Life, an intervention to reach and help inactive adults aged 50 years and older increase their physical activity. It was designed to fit within existing group-based structured physical activity programmes run by Local Sports Partnerships, thus maximising the likelihood of translation into policy and practice. Constructs from social cognitive theory, self-determination theory, and the conceptual model of group cohesion in exercise informed the conceptual model and the development of behavioural skills, social support, and group cohesion intervention strategies. Physical activity instructors supported by peer mentors, who also contributed to sustaining the intervention, implemented these strategies. Moving away from accounts of intervention development as a relatively simple linear process, we illustrate the complex interplay of theory, evidence, practice, and real-world contextual circumstances that shaped the development of Move for Life. Against this backdrop, we discuss issues relevant to the planning and reporting of behavioural and physical activity interventions in public health.