This paper presents new data for historic vein-hosted copper sulphide deposits in the Upper Palaeozoic Munster and South Munster Basins of southwest Ireland. Detailed mapping, 3D modelling, fluid inclusion microthermometry and geochronology from the Allihies area of the Beara Peninsula, have led to a new interpretation of the timing and development of ore mineralisation. Macro- and microstructural studies reveal that the ore-bearing, mainly E-W striking quartz veins are directly related to early extensional, basinal normal faults. Molybdenite Re-Os dating of the main-stage Cu lode yield ages from 367.3 ± 5.5 to 366.4 ± 1.9 Ma. This early vein system experienced subsequent late Carboniferous Variscan deformation, including cleavage development, sinistral SW-NE strike slip faulting, cataclastic deformation and recrystallization. The new timing of Cu mineralisation in SW Ireland has major implications for its relationship to the base metal deposits of the Irish Midlands.