The Irish agricultural sector now faces the challenge of increasing its productivity while maintaining and enhancing its environmental sustainability. As a consequence, areas of drained grassland are likely to expand in the coming years, especially in under-utilised or marginal lands near milking parlours, while former drainage systems will also be updated. An installed drainage system typically results in large mixed nutrient losses to water with further greenhouse gas (GHG) emissions to the atmosphere. Such losses may be mitigated by designing adapted drainage systems and installing in parallel remediation technologies (e.g. end-of-pipe bioreactors), but this requires a strong knowledge of the physical and chemical processes controlling water quality and GHG emissions in such systems. This paper investigated emissions and losses from four non-grazed plots recently converted from marginal land to grassland in the southeast of Ireland. Results showed that plots with a shallower water table and smaller size had greater water and nutrient losses in overland flow per unit of land area, while such losses were less in subsurface drains. Dissolved organic nitrogen dominated, but dissolved inorganic nitrogen was more abundant in the drains. Particulate phosphorus generally dominated in drains, except in plots with a shallow water table where dissolved unreactive phosphorus (DUP) was more abundant. In overland flow, a shallower water table resulted in a switch from dissolved reactive phosphorus (DRP) to DUP. Fertilization strongly increased P losses in overland flow, with DRP dominating. In contrast, nitrous oxide (N₂O) emissions were not impacted by differences in plot size or changes in water table depth across plots; nevertheless, within the same plot, they generally increased downslope with decreasing water table depth. These results highlight the importance of an integrated assessment of the controls on nutrient losses to water and the atmosphere to design drainage systems and remediation technologies in grasslands.