Changes in society demand constantly evolving education systems. Teachers' practice is crucial to the successful implementation of educational reforms. A major obstacle for practicing teachers in implementing reform mathematics is that they have not experienced it themselves as learners. Consequently, teachers require extended, practice-based professional development to support them in the implementation phase of reform endeavours. Japanese Lesson Study has increased in popularity internationally as a professional development model due to its potential to facilitate teachers to learn in and from the practice of teaching. This self-study examines two mathematics teacher educators' first experience of facilitating Lesson Study with Irish primary in-service teachers. Reflection on the Lesson Study carried out with in-service teachers in 8 urban schools revealed three critical incidents that promoted deeper understanding of the context. The findings provide insights into the realities of managing and leading Lesson Study with in-service teachers, in particular the various barriers affecting teacher engagement. Alternative approaches to practice to address these challenges are also identified.