The first major Pb-Zn mine in Ireland, located near Tynagh in County Galway, operated from the early 1960s until the early 1980s producing almost eight million tonnes of ore from surface and underground workings. This study was undertaken to characterise the hydrogeology of the area, and to assess the status of the groundwater, and this is the first detailed groundwater study of the area. Data sets were compiled from various state agencies and field data collection included well surveying and mapping, hydraulic head measurement, in situ well testing and recovery of samples for analysis. Thirty-nine wells were surveyed and 20 of those were sampled between 2008 and 2013. Samples were analysed for major anion and cations and a suite of metals (incl. Fe, Mn, Ba, As, Ni, Zn, Pb, Cu and Cd). The results are presented and discussed in the context of the geology, hydrogeology, mineralisation and regulations. A limited number of wells hydraulically downgradient of the mine site show consistent and persistent concentrations of elements that are above threshold levels or regulatory standards. Further, these wells are associated with the North Tynagh Fault, a deep seated, high angle normal fault that trends west-east for approximately 10 kilometres. This fault is shown to have been a critical factor in the deposition of mineral deposits in the Carboniferous period, and to have facilitated movement of acidic water that decalcified and weathered several million tonnes of rock during the Tertiary. This fault is now a key element of the local hydrogeology: significantly more groundwater moves through this discrete feature than through the mass of regional rock. Statistical analyses of the data sets using PCA, Q-Mode and Hierarchical Factor Analysis, and Trend Analysis confirm the importance of this structural feature and highlight specific relationships between various elements found in the groundwater samples.