Based in a time of major curriculum reform, this article reports on a qualitative case study of teacher professional development (PD) in the Republic of Ireland (ROI). Five mathematics teachers in an Irish secondary school were introduced to and participated in successive cycles of school-based lesson study (LS) over the course of one academic year. The research investigated how teachers' pedagogical practices and beliefs on student learning, specifically related to a revised curriculum, were impacted as a result of their participation in this model of PD. Data were generated through audio and field recording of teacher LS meetings, individual teacher interviews, teacher notes, samples of student work, observation of research lessons and researcher field notes. Analysis suggests that due to their collaborative planning, teaching, observation and reflection of research lessons, teachers began to incorporate and develop new pedagogical practices both inside and outside LS. This study suggests that in the introduction of centralised curriculum reform, LS can act as a powerful model of PD which can encourage the introduction of new pedagogical practices. This research also provides evidence for the introduction of LS as a viable form of PD in secondary schools in the ROI.