Marginalised groups of older people remain neglected in positive health and ageing (PHA) agendas, whether they concern healthy or active ageing concepts. Questions exist around the meaning of such constructs and the factors that enable disadvantaged populations to achieve equitable later-life experiences. Focusing on two such groups in Ireland, this study investigates the constituent dimensions of PHA for older Irish Travellers and older people who have experienced homelessness and the role of life-course and structural determinants in constructing PHA trajectories for these groups. The study involves a qualitative, participatory voice-led methodology, with analysis based on 49 in-depth life-course interviews with people aged 50 years and over from the two populations. Five interconnected dimensions of PHA are identified and presented. Four determinants related to life-course experiences and structural factors are identified as contributing to these dimensions: social relations, material and accommodation circumstances, formal supports and systems, and critical transitions and resilience. While illustrating the validity of PHA agendas for these groups when understood through their lived experiences, the findings highlight the significant deprivations and risks to rights that must be accounted for to secure meaningful gains in PHA for the groups.