Background: Schools are well recognised as critical contexts for the promotion of mental health and well-being and offer the potential to reach a large number of children at an early age, at a time when they are developing important attitudes and behaviours that may influence their future health. Ensuing from a number of large-scale epidemiological studies which have found that young Irish people are significantly struggling with their mental health and well-being, the Government of Ireland has committed to prioritising mental health promotion in schools through a multicomponent, preventive, whole school approach. Objectives: To obtain a detailed and integrated account of the implementation of a whole school approach to mental health and well-being promotion in the Irish primary school context. Method: A qualitative case study approach was undertaken employing in depth semi-structured interviews, document analysis and field notes. Thematic analysis was applied to all interview data and emerging themes were mapped on to the Theoretical Domains Framework (TDF) domains. Setting: Three Irish primary schools. Results: Facilitators and barriers that emerged from the data related primarily to the schools' economic and political context, organisational context, and internal and external partnerships and relationships. Conclusion: This study has demonstrated that the implementation process in 'real-world' practice settings, such as schools, is dynamic, complex and exists within a multi-level system and requires careful consideration of the numerous ecological factors that can influence implementation.