Salmonid populations were sampled in 36 streams, at altitudes between 100m and 200m, in counties Cork and Kerry, south-western Ireland. The catchment land use ranged from open moorland and rough pasture/improved grassland to afforested sites with varying levels of catchment afforestation, mostly coniferous but with some patches of mixed coniferous and deciduous trees. The main geology types within the region were slate, Old Red Sandstone and limestone. Study sites were electrofished, and the density, biomass, age structure and condition of salmonid populations were assessed. A range of in-stream riparian and catchment environmental variables were measured, and the data were analysed using stepwise multiple regression to identify the most influential environmental factors affecting trout metrics following principle component analysis. The majority of salmonids were trout (86%), and mean trout density ranged from 0.662 fish m -2 to 0.984 fish m-2 for all sites, irrespective of catchment geology. Trout condition did not differ significantly with level of catchment afforestation or geology, with the exception of limestone with high catchment afforestation. At sites with underlying limestone, trout condition decreased with increased catchment afforestation. River habitat structure was identified as the most important variable influencing trout density and biomass, irrespective of geology and water chemistry.