School leadership is a major issue in policy and practice because it can influence collaborative teacher professional learning and development. Systematic national research has indicated a significant dearth in knowledge of the influence of school leadership on teachers’ collaborative professional learning and development in primary schools. Furthermore, there has been little research into the overlap between school leadership and collaborative professional learning which are key components of school improvement and teacher professional development. Historically, school effectiveness research has focused on the principal as the ‘solo, heroic’ school leader. However, more recently school improvement literature, the international discourse and in turn national policy has shifted focus to the potential of distributed school leadership for school improvement to influence collaborative teacher professional learning and development in schools. This thesis presents key findings of an exploration into the influence of school leadership on collaborative teacher professional learning in Irish primary schools. A multi-site case study of four Irish primary schools was employed to explore the central phenomenon. The findings illustrated that shared school leadership for school improvement was influencing collaborative teacher professional learning and development in schools in multiple ways. The practice of shared leadership for school improvement was more reflective of delegation rather than genuine distributed leadership. One of the critical findings was that collaborative teacher professional learning and development was a beneficial byproduct of school development and the enactment of school improvement policy rather than the central aim of leadership activity. There was variance in the forms of professional learning within and between schools. School leaders were particularly reliant upon sustained support from the PDST to support collaborative professional learning and development for school improvement. This thesis concludes with a set of recommendations for future policy, practice, and research to progress enactment of the central phenomenon.