Due to the increased focus on school principals to develop leadership capacity as detailed in Irish educational policy (DES 2016, DES 2018, DES 2022), leadership development is seen as fundamental to the success of schools. The primary objective of this study was to investigate the relationship between school structures and the leadership development of staff in a post-primary context. More precisely, the following research questions were addressed: 1. Is there evidence of leadership capacity-building practices in the Irish post-primary context? 2. Can the culture of the school have an effect on the development of school leaders? 3. What type of school leadership builds leadership capacity? 4. Is there a relationship between the actions of a school STL and the development of leadership capacity within the school? 5. What factors can influence a school’s ability to build the leadership capacity of their staff? The research involved a thematic review of relevant peer-reviewed literature on leadership capacity-building and agile leadership. The concepts of the role of the principal, motivation, time, school culture and distributed leadership emerged from the review. A conceptual framework based on Looking at Our School – An aid to School Self Evaluation (DES, 2022) and the work of Lambert (2003) was developed to guide the study. Within this the researcher took a pragmatic approach to the research design, adopting a case study methodology within a mixed-method sequential process. The quantitative research instrument was distributed to all staff in two secondary schools. Following on from this the next phase consisted of a series of 15 semi-structured interviews with school staff in order to triangulate the findings. The research identifies recommendations for principals and Senior Leadership teams to develop sustainable and agile leadership capacity within the secondary school sector, with recommendations for further research and policy development in this area.