Trinity Access (TA) is a post-primary initiative, in the authors' university, that offers programmes for students, teachers and schools in areas of socio-economic disadvantage. The aims of the programme include helping students develop the knowledge, networks and skills needed to make informed decisions about future educational opportunities, and supporting teachers and schools to foster collaborative and reflective learning environments, which help students develop the academic capital they need to attain their aspirations. This is achieved by delivering programmes for students and professional development for teachers, embedded in an iterative cycle of data collection/analysis and feedback. A distinguishing feature of TA is its long-term commitment to its partner schools (20 years in some cases). This sustained engagement enables the depth and breadth of longitudinal research described herein. The research follows an observational study design within an overarching design-based research methodology. Data are collected annually from consenting students, teachers and management in 20 schools, enabling the researchers to establish students' levels of exposure/non-exposure to the intervention and to identify correlations between exposure and behaviours/aspirations. The goal of this paper is to provide an overview of the research design and methodology, illustrating its potential to deliver insights into practices that may alleviate educational inequalities.