Developing health literacy (HL) in youth populations, as a means to improving lifelong health behaviours and outcomes, has garnered increased interest in both research and practice in recent years. This mixed-methods process evaluation examines the acceptability, fidelity and dose delivered of the pilot of LifeLab Dublin: a nine-week, school-based HL intervention for low socioeconomic adolescents. The intervention, which comprised of two, out-of-school, interactive HL experiences in conjunction with seven school-based lessons, took place between September and December in 2021 and involved first-year students (aged 11–13) from one socioeconomically disadvantaged school in Dublin, Ireland. Data sources included focus groups, a questionnaire and self-reflection checklists. Overall, the intervention was perceived positively by the students and teachers, particularly the two interactive out-of-school experiences. The evaluation provided valuable insight into the refinements required to enhance the efficacy of the LifeLab intervention, as well as providing an evidence-base that can be used to inform future school-based HL interventions for low socioeconomic adolescents.