This paper explores the experiences and professional learning of a group of in-service primary school teachers, participating in a STEM education programme in Ireland. Four primary teachers from the same case school engaged in a two-year project to explore STEM teaching and learning. This STEM project was structured as a multi-tiered partnership involving these four in-service teachers collaborating with six STEM teacher educators and a group of pre-service primary teachers, working together to develop an effective school-university STEM collaboration. The project focused on developing and facilitating meaningful classroom-based STEM learning experiences for primary aged children in the school. This paper specifically explores the professional learning and experiences of the in-service teacher participants. The teachers engaged in co-creating and co-facilitating a series of STEM lessons over two years in partnership with, and supported by the university STEM Education faculty. The paper provides insight into the participating teachers' professional learning in STEM education, as well as implications for school-university partnerships.