This paper presents Irish-medium immersion (IMI) teachers' perceptions of their development of content knowledge of language (CK-L) and pedagogical content knowledge of language (PCK-L) as a result of engaging in form-focused professional development (PD) which combined both content knowledge and pedagogical learning outcomes. An intervention, trialling an explicit-inductive approach, was implemented across two stages. Twelve IMI teachers engaged in Stage One with students in Grades 5 and 6 in IMI settings (n = 274), and eight IMI teachers engaged in Stage Two with students in Grade 5 IMI (n = 188). Analysis of data from focus groups (n = 8) and semi-structured interviews (n = 11) show that targeted PD, researcher/teacher collaboration and provision of instructional materials enhanced teachers' CK-L and PCK-L with regard to specific grammatical forms in Irish and that teachers transferred their knowledge to both language and content classes. Implications arise for the provision of dual-focused PD for immersion teachers internationally.