Inappropriate, aggressive and disruptive behaviour in the classroom can be detrimental to child and teacher well-being. This study involved a longer-term evaluation of the Incredible Years Teacher [Classroom Management] programme (IYTP) undertaken mainly within disadvantaged schools in south-west Ireland. The IYTP is designed to strengthen teacher classroom management competencies. Eleven teachers, who had previously participated in a group-randomised control trial (RCT), took part in a 12-month post-baseline follow-up assessment. Psychometric and observational measures were administered to assess teachers when interacting with children (n = 217) in their classes. Qualitative data were collected from a sub-sample of teachers to explore their perceptions of programme impact and utility. The quantitative results showed some significant improvements in teachers’ classroom management, although few changes were recorded on teacher-child observation measures. Qualitative findings indicated continued implementation of proactive disciplining strategies and higher levels of teacher self-efficacy. These results suggest, albeit tentatively in view of the study limitations, that the IYTP can benefit teacher classroom management, as well as teacher well-being, particularly within disadvantaged schools where behavioural problems tend to be more prevalent. The IYTP could prove useful in improving teacher competencies and the classroom environment in the longer term, although a need for further research is indicated.