The Ox Mountains granodiorite (western Ireland) is situated along the Fair Head—Clew Bay line, a major Caledonian structure. The pluton was emplaced synchronously with respect to deformation on a major strike-slip shear zone system and experienced a pervasive episode of sinistral shear. Remnants of magmatic state deformation such as phenocryst alignments and tiling fabrics are present. An early phase of strain partitioning produced sinistral offsets on internal contacts. These structures were overprinted by a solid state fabric characterized by crystal plastic deformation in feldspar and the operation of recovery processes during quartz deformation indicating temperatures in the range 450–500 °C. A sinistral shear band fabric is ubiquitous and its formation was enhanced by myrmekite production leading to the formation of fine-grained feldspar and quartz. Late deformation partitioning during further pluton cooling produced a conjugate shear zone system containing mylonites. Deformation within the Ox Mountains granodiorite provides important constraints on the nature of late Caledonian age mid-crustal deformation along the Highland Boundary fault. The pluton also provides exceptional examples of the range of structures that may form in a syntectonic pluton.