Lesson study was introduced in two secondary schools in the Republic of Ireland (ROI) at a time of mathematics curriculum reform . The research investigated lesson study as a model of professional development which might support teachers in implementing the revised curriculum. Twelve teachers participated in the research and data included transcripts of lesson study meetings, individual interviews, samples of student work, and research lesson plans. Findings suggest that, through their participation in successive cycles of lesson study, teachers were supported in: encouraging more student communication of mathematical thinking, developing their roles as facilitators of student learning, and incorporating contextualised mathematical tasks. The research indicates that lesson study can be utilised in the introduction of curriculum reform and provides evidence of the viability of school-based lesson study in the ROI.