The key traits of growth, survival and maturation during the marine phase define the phenotypes of returning adult anadromous brown trout (Salmo trutta) and thus their population dynamics, stock structures and the characteristics of their fisheries. This paper reviews recent and historical evidence for spatial and temporal variation in life history traits and other demographic features of sea trout around the Irish Sea and examines some of the causal factors, focussing on the marine environment. Stock abundance, indexed by rod catches, showed synchronous variation over wide areas, suggesting a response to common factor/s. Contemporary data, based on scale reading and size structures, were compared with historical information to describe variation in growth, survival and maturation (as indexed by first return). Spatial patterns in growth and survival were different between eastern and western seaboards, with slower growth and lower marine survival characterising several rivers of eastern Ireland. North-south variation in growth was evident on the Wales to Scotland seaboard and tentatively linked to temperature gradients, although growth opportunity will be influenced also by the variable and complex patterns of marine productivity in the Irish Sea. Previous research focus has been on the role of freshwater factors in initiating anadromy as a tactic to optimise fitness. These results show the additional influence of marine habitat on life histories and consequently the age and size structures of sea trout stocks. Long-term changes in life history traits are described that might reflect marine climate change. The results look at sea trout from a marine ecosystem perspective in the Irish Sea and are discussed briefly in the context of sea trout fisheries and conservation management.