Bimodal Silurian igneous activity in SW Connacht occurred during basin subsidence and sedimentation, and before basin deformation. Mafic sills averaging 3 m thickness were preferentially intruded into and supported by a thin horizon of fine-grained siliceous sediment (Tonalee Fm.). Multiple injection produced the thicker sills. A primary igneous lithology of augite–plagioclase–olivine–phyric dolerite has, in some sills, been obliterated by pervasive hydration and carbonation expressed in a biotite-calcite–quartz–albite mineralogy. This represents volatile influx into magma at a sub-intrusive level, and contrasts with syn-intrusive assimilation and post-intrusive alteration that introduced silica and potassium into, and removed magnesium from marginal dolerite. The SW Connacht magmatism occurred in an extensional tectonic setting, probably in a NNE–SSW structurally controlled basin, and was coeval with bimodal magmatism in other parts of Ireland and Britain on both sides of the Iapetus Sutures.