Lower Carboniferous (Mississippian) limestones exposed on the northeastern margin of the Dublin Basin comprise syn-rift, shallow marine carbonate platform, and deep-water carbonate turbidite sequences. Multiple generations of fault- and fracture-related carbonate and quartz veins cut the strata and are locally associated with massive dolomitization of adjacent limestones. Associated saddle dolomite cements display inconsistent cathodoluminescence stratigraphies despite a common paragenetic context. Fluid inclusions record interaction of two end-member paleofluids: a moderate- to high-temperature (Th ∼ 150 to 300°C), low- to moderate-salinity brine, and a low-temperature (Th ∼ 60 to 135°C), high-salinity brine. Isotope analyses of host limestone, dolomite, and carbonate veins define trends of δ18O values from ∼ –3 to –22‰ VPDB, δ13C values from +4 to –4‰ VPDB, and 87Sr/86Sr values from 0.7080 to 0.7105. The wide ranges in δ18O and 87Sr/86Sr values of calcite veins reflect incursion of highly evolved waters that interacted to varying degrees with more radiogenic rocks along their flow paths and with the host limestones into which they were emplaced. Field evidence, including orientations of vein- and dolomite-associated faults, and presence of reworked zebra dolomites in Brigantian hanging wall debris-flow conglomerates, support a Viséan age of mineralization. Comparison with published data from the Irish Midlands and central Dublin Basin supports a conceptual model of syn-rift fluid flow where regional saline paleoaquifers were intercepted by localized hydrothermal cells on active faults, some of which extended into lower Paleozoic basement. Episodes of structurally controlled hydrothermal fluid flow, cooling and mixing of fluids, and variable fluid–rock interaction led to a suite of similar diagenetic products but a diversity of compositions that are neither geographically nor stratigraphically correlated. This study presents a rare example of syn-rift diagenesis recorded in syn-rift limestones on the margins of a metalliferous basin.