The deterioration of water quality in Ireland over the past thirty years and the nutrient losses from agriculture are of particular concern to policy makers. Agri-environmental schemes (AES) are designed to improve the environment and water quality. One of the more common AES measures is the exclusion of cattle from watercourses. Cattle exclusion provides a number of benefits such as reduction in nitrogen, phosphorus and sediment run-off, riverbank stability and improved vegetation cover and biodiversity. Using GIS methods a new national farm boundary spatial database SLIDE (Spatial Land Identification Database for Eire) is created utilising national administrative data and a spatial data storage model. This is the first examination of cattle exclusion and its cost effectiveness at the individual farm level across a national population of farms. Benefits are measured in the form of reduced in-stream faecal deposits. The spatial distribution of cost-effectiveness at the sub-catchment level highlights the differences between areas in terms of costs, benefits and extent. A cluster analysis is used to aggregate results. This information is utilised to undertake an ex-ante examination of the implications of a national policy to restrict cattle access to waterbodies for specific categories of farmers, under the EU Nitrates Directive. The analysis of the clusters show that due to the reduction of faecal deposition in watercourses, fencing on medium and high intensity farms is similar to 20 % more cost effective relative to farms with lower livestock densities. This study makes an important contribution to the literature in broadening our understanding of the impacts of policy decision making at the individual farm level and also the costs and scale of AES measures at a national level.