Extant research highlights the prevalence of smartphone addiction (SA) and associated psychopathologies, including daytime sleepiness, undergraduate or school-aged students, and how self-determination predicts these behaviours. However, sparse research investigates these phenomena in adults with intellectual disabilities. Online surveys were used to determine SA risk, daytime sleepiness and self-determination scores in adults with intellectual disabilities in education. Mediational analyses found a weak positive relationship between SA and daytime sleepiness was not mediated by controlled and impersonal orientations. Cross-sectional analysis highlighted no differences in SA scores between people aged under 22 and over 21. A moderate positive relationship was found between SA, controlled and impersonal, but not autonomous orientations. Contrary to previous research with general populations, interventions reducing controlled and impersonal orientations, rather than enhancing autonomy, may be effective in reducing SA. Findings may assist future research investigating SA in people with ID, and in the Irish context.