Early Ordovician collision of the Lough Nafooey Arc, part of the Baie Verte Oceanic Tract, with the passive continental margin of Laurentia, peaking at ca. 475 Ma in Scotland and Ireland, was followed by subduction polarity reversal. This study examined Upper Ordovician-Silurian sedimentary rocks from western Ireland to see if collision was followed by renewed arc magmatism. Despite the scarcity of dated igneous intrusions between the Grampian (ca. 470 Ma) and Acadian (ca. 420 Ma) orogenies in Ireland, detrital zircons show a continuity of activity, peaking at 440–480 Ma implying no regional magmatic hiatus. Differences in zircon U-Pb age spectra highlight the isolation of basins in the southern Killary Harbour area from those north of the South Mayo Trough. These latter rocks were largely derived by erosion from Moine and Upper Dalradian sources. In contrast, the Killary Harbour Basin shows decreasing influence from the Dalradian after ca. 436 Ma and increasing influence of contemporaneous magmatic zircons. These were transported from sources along strike from the present NE, probably at the southern end of the Scandian Mountains in SE Greenland. The western Irish basins formed as pull-apart basins in a forearc setting and are analogous to Cenozoic pull-apart basins in Sumatra.