BACKGROUND: Over the last three decades in response to the growing burden of diabetes, countries worldwide have developed national and regional multifaceted programmes to improve the monitoring and management of diabetes and to enhance the coordination of care within and across settings. In Ireland in 2010, against a backdrop of limited dedicated strategic planning and engrained variation in the type and level of diabetes care, a national programme was established to standardise and improve care for people with diabetes in Ireland, known as the National Diabetes Programme (NDP). The NDP comprises a range of organisational and service delivery changes to support evidence-based practices and policies. This realist evaluation protocol sets out the approach that will be used to identify and explain which aspects of the programme are working, for whom and in what circumstances to produce the outcomes intended. METHODS/DESIGN: This mixed method realist evaluation will develop theories about the relationship between the context, mechanisms and outcomes of the diabetes programme. In stage 1, to identify the official programme theories, documentary analysis and qualitative interviews were conducted with national stakeholders involved in the design, development and management of the programme. In stage 2, as part of a multiple case study design with one case per administrative region in the health system, qualitative interviews are being conducted with frontline staff and service users to explore their responses to, and reasoning about, the programme's resources (mechanisms). Finally, administrative data will be used to examine intermediate implementation outcomes such as service uptake, acceptability, and fidelity to models of care. DISCUSSION: This evaluation is using the principles of realist evaluation to examine the implementation of a national programme to standardise and improve services for people with diabetes in Ireland. The concurrence of implementation and evaluation has enabled us to produce formative feedback for the NDP while also supporting the refinement and revision of initial theories about how the programme is being implemented in the dynamic and unstable context of the Irish healthcare system.