This paper examines the impact, since 2008, of an international philanthropic organisation on the ageing sector in the Republic of Ireland. Several methods were used in a sector-wide evaluation, including documentary analysis, one-to-one in-depth interviews, a cross-sectional survey of grant-holders, and collaborative work with Northern Ireland. There was a relative lack of strategic focus on older people before the philanthropic work began; evidence for policy and planning was limited and advocacy was small-scale and largely uncoordinated. This may have been because the Republic of Ireland had the lowest proportion of people aged ≥65 in the EU. The findings show that philanthropy has played a pivotal role in enhancing capacity, infrastructure and expertise through large-scale investment in research and training, strengthening older people’s organisations and developing centres of excellence. Important initiatives include a national longitudinal ageing study, a commitment to develop a positive ageing strategy and roll-out of the Age Friendly Counties programme. The sustainability of these notable achievements relies on a sector-wide collaborative ethos, translating evidence into practice, actively involving older people and securing support from the academic, health and government sectors. Overall, the philanthropic organisation has contributed significantly to Ireland’s efforts to develop innovative, evidence-based ageing strategies and policies.