High health literacy (HL) is associated with improved health behaviours and outcomes. Schools are an ideal environment to improve adolescents' HL levels. This study assessed the feasibility of LifeLab Dublin, a co-designed HL intervention, for socioeconomically disadvantaged schools. Students (N = 356) and teachers (N = 15) from four Dublin schools participated in this study. A pre-post follow-up, single-arm feasibility, mixed methods study design was adopted, guided by the Orsmond and Cohn (2015) feasibility objectives. Data sources included research records, semistructured interviews and focus groups, and student and teacher surveys. A combined inductive-deductive reflexive thematic analysis was conducted on the interview and focus group data. Descriptive statistics were run on survey data. Student surveys measured HL, wellbeing, and health-related quality of life at baseline, postintervention, and follow-up. Data sources were analysed independently and integrated during write up. Interviews and focus group data indicated positive experiences with the LifeLab programme among students and teachers. School-level recruitment rates were low (6% participated), but the individual level was strong (91% participated). Attrition rates were 13.8% postintervention and 16.8% at follow-up. Including additional content for each health topic, flexibility in delivery, and the inclusion of structured teacher training may improve the programme. Moderate HL levels and good wellbeing was observed at baseline. Mean increases were observed from baseline to follow-up in students' HL, wellbeing, and health-related quality of life. Evidence supports the feasibility and acceptability of LifeLab for young adolescents and teachers, with progression to a definitive trial as a logical next step.