Dissolved inorganic arsenic (As), with epidemiological associations to a variety of cancers, is an emerging concern in some Irish groundwaters. Notable exceedances of the World Health Organisation (WHO) and Irish Drinking Water Regulations limit of 10 μg L-1 have been identified across the island. In Co. Louth, As is elevated (up to 60 μg L-1) in some private wells that intersect a fractured bedrock aquifer close to the contact between Silurian greywacke units of the Longford-Down Terrane and the Palaeogene Slieve Gullion igneous complex. No anthropogenic source of this As could be identified. Two drill cores recovered during late 2015, have been studied to identify potential geogenic sources and/or sinks of As. Bulk geochemistry (ICP-MS and ICP-AES) of drill core samples of granite, greywacke and basalt were taken along the cores. An average As concentration of c. 3 mg Kg-1 was found for greywacke samples and c. 0.4 mg Kg-1 for the granite samples, similar to expected upper continental crustal averages. However, intrusive basaltic dykes were found to contain up to 80 mg Kg-1 of As, elevated compared to the global basalt average of c. 2 – 5 mg Kg-1. Disseminated arsenic-bearing sulphide minerals (c. 5 μm) with associated cobalt (Co) and nickel (Ni) have been identified within the basaltic dykes using SEM-EDX as the likely source of arsenic in the groundwater. Iron oxy-hydroxide coatings along low-angle fractures within the granite, which can act as adsorption surfaces for As under oxic conditions, may also indicate several potential mobilisation mechanisms in this area.