A growing body of evidence supports the need for enhanced professional development (PD) in behaviour support in Irish schools. This study investigated the impact of a bespoke behaviour support package on the positive behaviour support (PBS) skills of teachers and Special Needs Assistants (SNAs) (N=17) in a special school for Autistic students with cooccurring intellectual disability (ID) and other complex needs. It also examined the impact of the PD on classroom practice. This complex mixed-methods case study consisted of two phases: Intervention and Implementation. The Intervention phase involved six 90-minute workshops incorporating behavioural skills training (BST) as a PD mechanism. BST, a competency-based training model, has been successfully applied in the PD of a broad range of front-line staff, but its evaluation in a study involving Irish educational professionals is a unique aspect of this research. During the workshops, participants rehearsed skills related to a customised behaviour support curriculum (BSC) with strands in choice-making, functional communication, and tolerance of uncertainty. Each unit of the BSC was an amalgamation of multiple socially valid evidence-based practices (EBP) recommended for the education and support of Autistic students with co-occurring ID. Skill development was measured through trials-to-criterion, alongside a baseline performance assessment for each BSC unit. The acceptability of BST-based PD was assessed in a post-intervention survey. The implementation phase of the study involved reviewing teacher planning documents and conducting semi-structured interviews. These methods were employed to assess the influence of the PD on classroom practices, explore participants' experiences of this form of PD, and determine what factors influenced implementation. Skill mastery was achieved in each of the nine BSC units. Participants also evaluated the PD positively. BSC units were incorporated into support plans for 14 of 16 students, though implementation varied across classrooms. The semi-structured interviews corroborated the positive survey feedback, highlighting the practicality of the training, the applicability of the content for the student cohort and the advantages of whole school teams attending joint PD. Key factors that impacted implementation were classroom leadership and the availability of follow-up support. The findings evidence how a bespoke evidence-based behaviour support curriculum and the corresponding skills can be imparted to whole school teams supporting students with complex learning and behaviour support needs.