There is some evidence that people with intellectual disabilities who live in the community are exposed to the same risks of alcohol use as the rest of the population. Various interventions have been evaluated in the general population to tackle hazardous or harmful drinking and alcohol dependence, but the literature evaluating interventions is very limited regarding intellectual disabilities. The National Institute for Health and Clinical Excellence recommends that brief and extended brief interventions be used to help young persons and adults who have screened as positive for hazardous and harmful drinking. The objective of this trial is to investigate the feasibility of adapting and delivering an extended brief intervention (EBI) to persons with mild/moderate intellectual disability who live in the community and whose level of drinking is harmful or hazardous. The study has three stages, which include the adaptation of the Extended Brief Intervention (EBI) for people with intellectual disability, a single blind, randomised controlled trial of an individual Extended Brief Intervention to test the feasibility of the intervention, and a qualitative study that will assess the perceived acceptability and usefulness of the intervention. Fifty participants in total will be recruited from community intellectual disability services and social care or third sector organisations. The main outcome is a reduction in alcohol consumption measured by the Alcohol Use Disorders Identification Test. Alcohol misuse is a relatively under-researched mental health problem in people with intellectual disabilities. Therefore, the study addresses both diagnostic issues and the delivery of a simple first stage intervention, which is available to the population of average intelligence and young persons in particular. The findings from the study will guide the preparation of a large-scale study to test whether this treatment is clinically and cost-effective in this population. ISRCTN58783633 (19 December 2013).