Irish children's physical activity (PA) levels are below national guidelines. Schools are an ideal setting to promote PA but it remains challenging. This study evaluates the effectiveness of a step-back approach to Project Spraoi (PS); a facilitator (Energizer) led school-based PA intervention. Each Energizer in year one had 2 contact days per week with all classes (n = 11). A 'step-back' progression, with 50% less Energizer contact time, in year two and no Energizer contact in year three, was adopted. Objectively measured PA and cardiorespiratory fitness (CRF) were measured annually with the same children (n = 85). Stakeholders (children (n = 287), teachers (n = 11) and other school staff (n = 8)), participated in process evaluation data collection. After an initial increase in year one, PA levels declined in year two and three; yet all children were more active at intervention end compared to baseline (p < 0.05). The biggest barriers to implementation were a lack of time and 'lack of Energizer contact', with the Energizer cited as the most frequently identified facilitator by all stakeholders. This study outlines the importance of a facilitator in the long-term success of health promotion intervention delivery in a primary school context.