The aim of this study was to initiate a co-design process with adolescents to inform the development of a targeted health literacy intervention for implementation in designated socioeconomically disadvantaged post-primary schools in Ireland. Purposely developed vignettes were explored in a series of eight workshops that were conducted separately with staff (n = 26) and students (n = 33) across four schools. Data was analysed using content analysis. A number of key health topics were identified as important and influential for the participants in this context: food choices, mental health and wellbeing, physical activity and sedentary behaviour, sleep and substance misuse. Participants also suggested many health-related capacity building actions. Participants recognized that many of these health topics and capacity building actions were intertwined and also highlighted that some of these actions may be more feasible and/or impactful than others. For example, students and school staff both indicated the need to use relevant, applied and engaging approaches to improve health literacy and subsequent health behaviour. The co-design process adopted empowered stakeholders to actively engage in the design and development of future intervention strategies, which may increase the likelihood of acceptability, effectiveness and sustainability of the resulting intervention.