Background: Promotion of good nutrition is essential for reducing the risk of chronic disease and premature death. Evidence shows menu labeling interventions should be implemented in workplaces as part of a comprehensive approach to improve employees' dietary habits; however, implementation challenges have arisen. This article describes a protocol for a multiple case study to explore the factors that impact on implementation of a calorie menu labeling policy in Irish public hospitals. Methods: Using a multiple case study design, comprising four Irish acute public hospitals, this study will draw on multiple perspectives and sources of evidence (observations followed by interviews, focus groups, and documentary analysis) to allow for a comprehensive depth and breadth of inquiry. Data collection and analysis will be guided by the Consolidated Framework for Implementation Research, bringing together constructs from implementation theories to understand the complexity of implementing policies. Hospitals will be categorized into high and low implementers of the policy based on quantitative data obtained from structured observations. Using framework analysis, within- and cross-case analyses will be performed to identify factors influencing policy implementation and to identify distinguishing patterns across high and low implementers and across hospital direct and indirect stakeholders. Strategies will be employed to ensure rigorous case study research, for example, triangulation, audit trail, reflexivity, and thick descriptions. An integrated knowledge translation approach, where researchers work with stakeholders throughout the research process, will be adopted to facilitate the translation of research into policy and practice. Discussion: This protocol highlights methodological insights in utilizing case study research to gain a greater understanding of the menu labeling implementation process. Study findings will be relevant to policy makers and other stakeholders involved in the rollout of such interventions and will provide a foundation to select and tailor implementation strategies to assist with scale-up of calorie menu labeling across the health service.