The marine feeding ecology of sea trout (Salmo trutta) and information on the prey species they consume is fundamental to understanding the factors likely to influence their distribution, migration and growth in the marine environment. This paper presents stomach content analysis from 991 marine-caught sea trout, sampled from around the Irish Sea, in littoral and offshore pelagic habitat. Frequency of occurrence and percentage composition by weight were used to compute an index of prey importance. Fish, which was the major dietary component, was dominated by two components, sand eels (Ammodytes spp.) and sprat (Sprattus sprattus L.). Sand eels were the most prevalent with percentage occurrence and percentage abundance by weight of 56% and 62% respectively, followed by sprat at 28% and 18% respectively. Spatial and ontogenic variation was observed and pelagic feeding fish, which were almost exclusively feeding on sprat, demonstrated significantly higher feeding intensity compared to inshore feeding fish. Identifying the keystone prey species contributes to identification and understanding of the distribution of important sea trout habitats and, in the longer term, to development of enhanced management and conservation strategies for sea trout and its marine habitat, their prey and associated habitat types.