Shiga toxin-producing Escherichia coli (STEC) is a zoonotic pathogen causing human infections, ranging in severity from asymptomatic infection to haemolytic uraemic syndrome. STEC infections are commonly associated with O157 and O26 serogroups, with waterborne transmission representing a key exposure route in Ireland. We sought to investigate STEC concentrations in surface waters and groundwater wells across the Corrib catchment over an 11 month period. 19 sites comprising river (n=5) and groundwater (n=14) sources were sampled fortnightly between May 2023 and April 2024 (417 samples). Colilert-18® and quantitative polymerase chain reaction (qPCR) were employed to detect and quantify total coliforms, E. coli, STEC O157 and O26. E. coli was detected in 262/417 (63%) samples, including 115 (44%) river and 147 (56%) groundwater samples. At least one STEC serogroup appeared in 277/417 (66%) samples and in 177/262 (68%) of E. coli positive samples. Overall, 244/417 (59%) samples tested positive for STEC O157, comprising 49/115 (43%) river and 195/302 (65%) groundwater samples. STEC O26 was detected in 97/417 (23%) samples and was more prevalent in rivers (n=29, 25%) than groundwaters (n=68, 23%). Weak correlations were observed between E. coli concentrations and mean monthly rainfall (r = 0.159, p = 0.001), STEC O157 and mean monthly temperature (r = 0.171, p < 0.001), and STEC O26 and mean monthly rainfall (r = -0.174, p < 0.001). This study highlights the dynamics of STEC in surface and groundwaters within an Irish catchment, providing insights for improved source protection and risk management of drinking water supplies.