Supported decision-making (SDM) – where people use trusted friends, family members, and professionals to help them understand the situations and choices they face, so they may make their own decisions – is a means for increasing autonomy. This research aimed to explore the views and experiences of adults with intellectual disabilities in Ireland and their decision-making supporters (family and professional carers) regarding SDM in order to inform the development of recommendations for guidelines to facilitate SDM for this cohort in anticipation of the commencement of Assisted Decision Making (Capacity) Act. A mixed methods systematic review identified a gap in current understanding of how adults with intellectual disabilities and their supporters use SDM to make everyday decisions. A qualitative survey exploring family and professional carers’ experiences of providing decision-making support during the COVID-19 pandemic indicated that carers struggled to facilitate decisional support for adults with intellectual disabilities while restrictions were in place. An environmental scan of online Irish resources on SDM for adults with intellectual disabilities and their supporters identified a paucity of resources relating to the application of SDM in everyday decision-making. Focus groups conducted separately with four stakeholder groups (adults with intellectual disabilities, family carers, professional frontline staff and professional supervisory staff) explored aspects of SDM each group valued most. Two World Cafe-style multi-stakeholder workshops were held to show focus group participants the preliminary focus group conclusions and obtain further thoughts and perspectives on SDM. All findings across this project were synthesised into five key recommendations for the construction of a guide to SDM. This thesis contributes to the literature by providing a multi-stakeholder perspective on the most valued aspects of SDM for adults with intellectual disabilities and their decision-making supporters in an resource to be designed for their benefit.