The implementation of models of integrated care for chronic conditions is not well understood. We conducted a realist evaluation to determine how and why the implementation of the National Diabetes Programme in Ireland worked (or not). This study showed that, locally important contexts facilitating implementation included staff experience of delivering diabetes care, capacity, and familiarity with the intended purpose of new clinical posts. The extent to which integrated care was adopted and implemented depended on judgements made by professionals working in these contexts; specifically, judging the relative advantage of the programme and whether to engage in negotiations to legitimize their new roles in diabetes care.