This paper analyses the likely impacts of changes in climate for nine hydrologically diverse catchments throughout Ireland. When assessing the impacts of climate change on water resources there is a cascade of uncertainty that begins with the establishment of future pathways of development and ends with impact assessment (Wilby, 2005). In order to represent uncertainty in future simulations, statistically downscaled output from three Global Climate Models (GCMs), forced using two emission scenarios is used to force a lumped, conceptual rainfall-runoff model for three future time periods; the 2020s, the 2050s and 2080s. Changes in catchment storage, streamflow and extreme events are assessed through comparison with the GCM modelled control period 1961-1990. Future simulations suggest that reductions in soil moisture storage throughout the summer and autumn months are likely for each catchment. The extent of decreases are largely dependent on the storage potential of individual catchments; the lower the capacity of catchments to store water, the greater the sensitivity to climate change. Reductions in groundwater storage during the recharge period will increase the risk of severe drought, as the failure of winter or spring precipitation may result in prolonged drought periods where the groundwater system is unable to recover. Greatest reductions in streamflow are likely for the autumn months in the majority of catchments, while greatest increases are suggested for the month of February. The magnitude and frequency of flood events are shown to increase, with the greatest increases associated with floods of a higher return period. Uncertainty in future simulations derived from HYSIM parameter uncertainty is found to be more important than uncertainty due to emission scenario.