Background: The prevalence of features of attention deficit hyperactivity disorder (ADHD) in children with intellectual disabilities (ID) in Irish schools is unknown. The aim of this study was to examine the prevalence of features of ADHD in a special school, in order to ascertain the number of children who may need further assessment for ADHD. The study also explores the reliability of the Conners Teachers Rating Scale in this population. Method: All teachers in a special school for children with ID were asked to complete the Conners Teachers Rating Scale and the Attention-Distractibility, Inhibition-Excitation Classroom Assessment Rating Scale, for those children whose parents had consented for them to take part in this study. Consent was obtained for 84 children a response rate of 71%, between the ages of 5 and 18 (mean = 10.5 years; SD = 3.7). Results: The Conners Teachers Rating Scale was found to be internally reliable and had a normal distribution with our results. Overall, 55.9% of participants (4784) had markedly elevated scores (T greater than 69) for at least one of the target subscales, which were the "Hyperactivity", "Inattention" and the "ADHD Index" subscales of the Conners Teachers Rating Scales. In addition, the findings would suggest that the Conners Teachers Rating Scale can be a useful screening tool in the population of school children with ID. Conclusions: The study suggests that ADHD may be under diagnosed in children with ID. This has practical implications for the mental health needs of these children. It is recommended that further studies are carried out to determine the prevalence of ADHD in this population.