Background According to data provided by the National Council for Special Education (NCSE), between 2020 and 2021, there was a 15% increase in the number of special classes specifically for learners with Autism/Autism Spectrum Disorder (A/ASD) attached to mainstream schools in the Republic of Ireland. Given the considerable growth in A/ASD classes in schools, examining the Teacher Self-Efficacy (TSE) of teachers working there enables us to identify strengths and areas for professional development (PD). Method This paper reports the findings from an Irish study into Teacher Self-Efficacy (TSE) of teachers working in special classes for learners with A/ASD. The study involved a large sample (n = 139) of teachers responding to an online questionnaire. The questionnaire included the Autism Self-Efficacy Scale for Teachers (ASSET). Results Analysis of the completed questionnaires reveals that the areas teachers feel least confident in are their abilities to train peer models, to teach play skills, and to translate assessment information into teaching objectives to the students. They felt most confident in their ability to use visual supports to foster student independence, to describe their student’s characteristics that relate to A/ASD, and to communicate and work effectively with parents or caregivers. Conclusions The findings of this study present implications for professional development for teachers of learners with A/ASD, namely that upskilling is needed in training peer models, play skills, and translating assessment information into teaching objectives. The results also add further evidence to the use of the ASSET as a reliable measure of assessing the Teacher Self-Efficacy for teaching students with A/ASD.