ABSTRACT Background Many Irish children are failing to meet the recommended guidelines for physical activity. Research shows that children are failing to develop the necessary physical literacy skills to reverse this trend. Early childhood has been identified as a critical period to intervene, with the school environment identified as a key environment to do so. Qualitative research is increasingly included in intervention studies to aid the development of acceptable, attractive, effective and sustainable interventions. The aim of this study was to evaluate the implementation process of the MWBW intervention (Exploratory trial) in the primary school setting, allowing the research team to assess the quality of the intervention’s implementation. Objectives included assessing whether the intervention is suitable for the primary school environment, and to inform for future, mainstream implementation through a comprehensive process evaluation. Methods The intervention is underpinned by both the theory of constraints and self-determination theory, and is designed using the comprehensive framework outlined by the behaviour change wheel. The intervention was delivered in 18 primary schools (22% DEIS) to 925 participants (age range 6–10 years, mean 7.55). The intervention was co-delivered by trained coaches and class teachers who were being upskilled (by trained coaches) whilst the intervention was being delivered. Qualitative data were gathered in the form of child focus groups (N = 32, 62% Male), teacher (N = 31) and principal (N = 5) questionnaires (open-ended questions), coach focus groups (N = 16, 81% Male) and weekly reflections. Data were analysed thematically following Braun and Clarke guidelines on using thematic analysis. Results Five key themes, with subsequent sub-themes, were identified: (1) Implementation of the Moving Well-Being Well model, (2) Outcomes from the intervention, (3) Fidelity of implementation, (4) Key components of favourite games and (5) Challenges faced and areas to develop. Conclusions Findings would suggest that the MWBW intervention is suited to the Irish primary school environment and its model of implementation can provide benefits to both children participants and teachers. Findings also show that the intervention has largely been delivered as intended showing potentially significant increases in participants overall FMS. Future research should examine the contextual impacts on the overall FMS improvement and how it varies from site to site. Evidence from this study also suggests that in order to produce similar or improved results in the future as part of a wider-scale roll-out of the intervention in schools, researchers should look to further assist teachers (possibly through added resources and/or training) to assist fidelity of implementation as well as looking at other factors that may influence intervention outcomes such as weather, seasonality, access to indoor facilities and environmental factors.