The early Paleozoic rocks of eastern Ireland span the suture zone between the Laurentian and Ganderian continental margins of the Iapetus Ocean. The Grangegeeth Terrane comprises a Laurentian continental fragment and Ordovician volcanic arc that formed the southern margin of the late Ordovician Rathkenny Basin of Moffat Shale facies mudstone. Together, these were overstepped in Wenlock time by Laurentiaderived greywackes and became the southernmost tract of the Longford–Down Terrane accretionary prism as subduction brought them into the Laurentian-margin trench. South of the suture, Middle Ordovician failed rifting of a Ganderian volcanic arc terrane was followed by shortening during continuing subduction under the north-facing arc. In Newfoundland, a Ganderian volcanic arc migrated across the ocean in mid-Ordovician times to accrete to Laurentia. In Ireland, the late-accreted arc on the Laurentian margin was formed on a Laurentian microcontinent, and Ganderia had an active margin throughout Late Ordovician time. Silurian closure of Iapetus was between the leading edge of Ganderia and the Laurentian margin, unlike in the Canadian Appalachians, where separate Ordovician and Silurian sutures are recognized. Iapetus was narrow by Katian time but subduction-related magmatism continued into Wenlock times on both margins.