Implementing inclusive education policy is a complex issue intrinsically woven into the complex fabric of teaching and learning. This research asks the question: how efficacious do teachers feel in translating inclusive principles into practice? Research supports the view that teacher efficacy - teachers’ perceptions of their own teaching competence - is one of the most important variables related to positive teaching behaviours and student achievement. Using the lens of social cognitive theory, this study examines teacher efficacy and explores: (a) the relationship between personal teacher efficacy (PTE), and general teacher efficacy (GTE), from the scales devised by Hoy and Woolfolk (1993), and (SEN) teacher efficacy from a self-designed scale; (b) the extent to which mainstream teachers’ believe that they have the knowledge, skills and competencies, following pre-service, to successfully include pupils with special educational needs; and (c) the influence of other contextual variables on SEN teacher efficacy. The study employed a quantitative approach to investigate the views of mainstream primary teachers in Ireland (N=244), who had qualified between the years 1998-2007 inclusively. Findings reveal a complex picture in relation to teacher efficacy with regard to pupils with SEN in mainstream schools. Teacher efficacy in relation to special educational needs - SEN efficacy - is unique and different from personal teacher efficacy (PTE) and (GTE) indicating that there are specific and additional knowledge, skills and competencies required to work in inclusive settings. While teacher preparation has a significant impact on efficacy, other contextual factors, such as intrinsic and extrinsic school factors, all serve to impact on teacher efficacy. These findings have implications for teacher educators, school principals, school support services and policy advisors.