Research has shown immersion to be effective, yet our understanding about the integration of language and content in Irish-medium immersion (IMI) pedagogy remains incomplete. This article reports on how the teaching of mathematics in the IMI elementary setting, supported pre-service teachers in bridging the language and content gap. The study utilised Japanese Lesson Study (LS) to design and implement mathematics lessons and qualitative data were collected from a variety of sources. Findings provide unique insights into the knowledge demands related to designing and implementing content lessons and reveal the challenges for pre-service teachers in providing balanced language and content instruction.