Petrogenetically related, Caledonian, calc-alkaline lamprophyric and shoshonitic dykes in eastern Ireland appear coeval with sinistral transtension. These dykes are exposed in a 10 km wide zone trending SW from Clogher Head, that strikes anticlockwise of the surface trace of the Iapetus Suture. Lamprophyre dykes intruded sinistral Riedel shears with a mean strike-trend at 040°. Riedel shears that host lamprophyre dykes strike anticlockwise of primary sinistral shears by 35°, consistent with sinistral transtension: primary shears strike 070°. parallel to the mean strike trend of shoshonitic dykes. Sinistral. primary shears may have controlled the emplacement of Caledonian, shoshonitic, intrusive rocks. Using lamprophyres from Clogher Head as examples, a new model is proposed for lamprophyre genesis and emplacement related to simple shear. In this model. transient decompression of metasomatized mantle during simple shear triggers release of volatile-rich potassic magma, which ascends to crustal levels along steep strike-slip faults. An association between lamprophyre magmatism and deep faulting suggests that relatively deep (>60km) seismicity may precede magma ascent. This model can account for several paradoxes of Caledonian magmatism.