The goal of this project is to examine how predicted changes to Ireland's climate will impact hydrology in nine river basins. Recent observations confirm increases in global mean temperatures, rising global average sea level and diminishing snow and ice cover (IPCC,2007). This warming, and the consequent rise in atmospheric water vapor have led to an increase in mean precipitation over northern Europe as well as an increase in the frequency of heavy precipitation events over most land areas. Rises in global average surface air temperatures are expected to continue in the 21st century. Furthermore, models suggest that hot extremes, heat waves and heavy precipitation events will continue to increase in frequency. To simulate past and future climate, boundary data from the European Centre Hamburg Model Version 5 (ECHAM5) global climate model are used to force the Rossby Centre Atmosphere Model (RCA3) regional climate model. This provides dynamically downscaled precipitation and temperature data under past and future climate scenarios. Based on the SRES-A1B scenario, temperatures are expected to rise throughout the year. Mean annual precipitation is expected to increase a little, but of greater interest is the significant shift in its seasonal distribution. The downscaled precipitation and temperature data from ECHAM5/RCA3 are used to force the HBV-light model. This conceptual rainfall/run-off model is based on HBV96, but its user interface facilitates Monte Carlo simulations. This feature is particularly useful during the calibration stage, and results in very good calibrations across all catchments. Simulated streamflow from nine catchments are analyzed to quantify how the catchments will respond to expected climate change. Catchments were selected to ensure variable size, topography, geology, land cover and climatology. Changes in the seasonal cycle of precipitation coupled with increased temperature result in dramatic changes to the seasonal cycle of streamflow. While the project was motivated by an interest in extreme flooding events, the combination of decreased summer precipitation and increased summer temperature is shown to result in a substantial increase in drought risk. Changes in storage and therefore early winter conditions vary by catchment, which impacts the extent to which catchments are susceptible to increased flood risk in winter. While this study focuses on Irish catchments, the method and results are relevant to studies concerning the impact of climate change on hydrology in general.