Background: Social, emotional, and behavioural difficulties in the classroom pose considerable challenges for teachers, and may impact the well-being of children and teachers alike. The Incredible Years Teacher Classroom Management (TCM) programme aims to promote socio-emotional child development, improve teacher-student relationships, and build stronger home-school links. Previous research suggests that the TCM programme leads to positive outcomes for children and teachers, but few independent studies have examined this programme outside the United States where it was developed. Fewer still have explored the experiences of those involved (both directly and indirectly) with TCM training or how participation on this programme can change the way teachers view, and engage in, classroom management. Objective: The aim of this study was to explore the experiences of all key stakeholders involved with delivery and receipt of the TCM programme in Ireland. Specifically, it purported to document the perceptions of teachers and school principals regarding behaviour management, and to examine the process of programme delivery from the perspectives of teachers and the TCM delivery team. Short- and longer-term analyses of both qualitative and quantitative data were used to examine aspects of programme delivery and its overall impact. Method: This mixed method process evaluation was nested within a randomised controlled trial of the TCM programme, one of the few independent stand-alone replications conducted to date. Data were collected over an 18-month period using a number of methods including in-depth interviews with key stakeholders (teachers, principals, and programme delivery team) (n = 23) classroom observations (n = 22), and self-report questionnaires (n = 11). Interview data were analysed using Framework Analysis, whilst quantitative data were subjected to a series of factorial and one-way repeated measures ANOVAs. Results: At baseline, teachers and principals experienced a range of challenging behaviours in school, which were managed with varying success. Post-training, teachers reported a high degree of satisfaction with the TCM programme, although some of the learning tools used were not regarded as helpful. Training was seen as beneficial in a number of ways, particularly by providing a new approach to classroom management and creating opportunities for peer-sharing and collaboration. The qualitative analysis suggested that the TCM programme reduced isolation, and empowered teachers by increasing self-efficacy and providing practical management tools. The frequency of teachers’ negative management strategies was significantly reduced, and their positive management strategies significantly increased. However, there was no significant change in their use of praise. Longitudinally, both the qualitative and quantitative analyses show that benefits to teachers, children, and classrooms were maintained and became embedded into routine practice. Conclusions: This study represents an original contribution to knowledge about the process of TCM programme implementation and participation. The findings have important implications for the applicability and utility of the TCM programme within Irish primary schools. They underscore the important role of teachers in fostering child socio-emotional development, and the need to support them in this frequently challenging responsibility. The findings suggest that the TCM programme provides an effective and acceptable continuing professional development programme for teachers, and that early investment in such programmes may reap considerable rewards in the longer term for both teachers and students alike.