Granophyres and microgranites of the Tertiary central intrusive complex of Slieve Gullion, NE Ireland are intimately associated with appreciably larger volumes of mafic rocks whose compositions extend between basalt and basaltic andesite. These felsic intrusions show near minimum melt compositions and are characterized by trace element abundances and ratios consistent with evolution by crystal fractionation within a suite of granite magmas. Rb, Cs, Th and U all increase while Sr and Ba and Eu decrease with differentiation, suggesting an important feldspar control on the development of this felsic suite. All rocks are enriched in light rare earth elements relative to heavy rare earth elements and show negative Eu-anomalies (Eu/Eu* = 0.5–0.05), the extent of the anomaly increasing with differentiation. Isotopic ratios of Sr and Nd (87Sr/86Sr57.6Ma = 0.70983–0.71044 and 143Nd/144Nd = 0.512379–0.512384) rule out an origin through straightforward crystal fractionation of basaltic magmas, and the Nd isotopes are distinct from those of the Scottish Hebridean centres such as Skye. The δl8O and δD values on whole rock and mineral separates show evidence for interaction between the granitoids and a meteoric groundwater system. However, the extent of the interaction is not as extreme as that in the Skye granitoids. We interpret the granites as end products of advanced crystal fractionation of mantle-derived mafic liquids which interacted with lithosphere. The isotopic distinction between our data and those from the Hebridean islands are a further argument for lithospheric provinciality in the NW British Isles.