In many countries significant amounts of public funding are devoted to supporting firms’ R&D and innovation projects. Here, using panel data on the innovation activities of Irish manufacturing firms we examine the legacy effects of public subsidies for new product development and R&D. We examine five alternative mechanisms through which such effects may occur: input additionality, output additionality, and congenital, inter-organisational and experiential behavioural additionality. Tests suggest contrasting legacy effects with R&D subsidies generating legacy output additionality effects while new product development subsidies have legacy congenital and inter-organisational behavioural additionality effects. Our results have implications for innovation policy design and evaluation.