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Graphical Abstract Summary: The aims of this study were to describe quarter-level prevalence of intramammary infection (IMI) and somatic cell count (SCC), to evaluate the performance of commonly used SCC thresholds for the diagnosis of quarterlevel IMI and determine alternative thresholds for diagnosis of quarter-level IMI as defined by a positive aerobic culture in late lactation. A total of 8,177 quarter-level milk samples were available for analysis from both primiparous and multiparous cows from 21 Irish herds. The overall prevalence of quarter-level IMI was 6%; higher for primiparous (10.2%) than multiparous (5.2%). The most common etiological IMI agent was Staphylococcus aureus . Sensitivity and specificity were maximized at SCC thresholds lower than the traditional 200,000 cells/mL (61,000 cells/mL for primiparous; 101,000 cells/mL for multiparous). The identified quarterlevel SCC thresholds should be evaluated as guidance for selective dry cow therapy in the Irish dairy production scenario.