IntroductionThere is a critical need for interventions that can be feasibly implemented and are effective in successfully engaging adolescent females in physical activity (PA). A theory-based, peer-led, after-school PA intervention, the Girls Active Project (GAP), was codesigned with adolescent females. This study aimed to assess the feasibility of implementing and evaluating the GAP programme.SettingOne single-sex, female-only, designated disadvantaged postprimary school (students aged 12–18) in Dublin, Ireland.MethodsMixed methods were applied with multiple stakeholders over a 12-week trial (March to May 2021). A single-arm study design was used to examine intervention: reach, dose, fidelity, acceptability, compatibility and context. Feasibility of using proposed self-reported outcome measures (moderate-to-vigorous PA levels, self-rated health, life satisfaction, PA self-efficacy and PA enjoyment) was also explored. Due to school closure resulting from the COVID-19 pandemic, the intervention was delivered both online and in person in the school setting.ResultsEight exercise classes were peer delivered by project leaders (n = 6, students aged 15–17) to intervention recipients (students aged 13-14). Recruitment was low (n = 8, 10% of eligible students, mean age: 13.3 SD: 0.46), yet retention was high (n = 7/8, 88%). Attendance rates were satisfactory (68%), and the intervention was implemented with high fidelity (87%). Data completion rates suggested proposed self-reported outcome measures were deemed appropriate (≥ 95%), except for weight (50%) and height data (80%). Despite COVID-19 hindering intervention implementation, both quantitative and qualitative data suggested that stakeholders were satisfied and perceived the in-person delivered intervention to be compatible with the school setting. Recommended refinements included extending class duration, introducing different rewards, and boosting programme awareness.ConclusionsFurther thought must be given on how to increase recruitment. Overall, the in-person delivered after-school PA programme was well-received by stakeholders and shows promise as an intervention that can be feasibly implemented and evaluated. Suggested improvements to the GAP intervention programme are recommended, before continuing to a more robust evaluation.Trial registration10.17605/OSF.IO/75HWJ (prospectively registered, date of registration: 9th December 2020)