Background A new legal capacity act was introduced in Ireland in 2015. This study aimed to identify and critically examine key issues in the area of decision-making capacity from the perspective of psychologists working with adults with an intellectual disability. Methods A qualitative exploratory approach was employed, and the study was positioned in a social constructionist framework. Purposive and snowballing sampling methods were used to recruit 15 clinical psychologists working with adults with an intellectual disability. Data were collected with the use of individual semistructured interviews. Interview transcripts were analysed using a model of thematic analysis. Results Six themes were identified: (1) a presumption of capacity but a culture of incapacity, (2) supporting decision making as a process, (3) authenticity of decision making, (4) need for support and training, (5) contributions of psychology and (6) the way forward. Conclusions Participants described that people with intellectual disabilities were often excluded from decision-making processes. They welcomed the functional approach to decision making, considered substituted decision making to be necessary within a support framework and described supporting decision making as a process. Systemic, resource and attitudinal challenges were identified.