The Ordovician elements in the Caledonides of western Ireland may be divided into the Clew Bay, South Mayo, North Connemara and South Connemara sub-zones. Evidence suggests the presence of a number of proximal suspect terranes, and the remnants of one, or possibly two, arc complexes. Palaeontological and sedimentary evidence suggests an allochthonous relationship between the Silurian rocks of Croagh Patrick and North Galway. These successions may therefore not represent a single sedimentary overlap to the Ordovician suspect terranes. End-Silurian deformation was the result of transpression, which was responsible in part for the present configuration of the area’s geology. Deep seated shear zones, with fuchsite and gold mineralization were foci for lateral translation concomitant with southward directed thrusting, northward directed back thrusts and variable degrees of metamorphism being of higher grade in the north. The end-Silurian event is intermediate between the high grade metamorphism in parts of the Appalachians and the non-metamorphic Silurian sequences of Scotland.