U-Pb geochronological data from the Connemara region of Ireland indicate that continental arc magmatism along the southern margin of Laurentia was short-lived, lasting only from ca. 475 to 463 Ma. Previous work has demonstrated that intrusive activity in Connemara was broadly synchronous with Grampian mid-crustal deformation and upper amphibolite facies metamorphism of Neoproterozoic–lower Paleozoic Dalradian Supergroup rocks. The two oldest intrusions, the Currywongaun and Cashel–Lough Wheelaun gabbros, have U-Pb zircon ages of 474.5 ± 1.0 and 470.1 ± 1.4 Ma, respectively, whereas the U-Pb xenotime age of the postdeformational Oughterard granite is 462.5 ± 1.2 Ma. Thus, the implied age of the Grampian orogeny in Connemara is substantially younger than generally acknowledged, but consistent with other age constraints from Ireland and Scotland. A revised age for the Grampian orogeny helps to solve a long-standing controversy about the age of contractional deformation and arc magmatism in Dalradian Supergroup rocks of the British Isles. This event now demonstrably postdates Neoproterozoic–lower Paleozoic rift deposits and Laurentian passive margin sediments, a finding consistent with deposition of Dalradian Supergroup rocks in an extensional environment at or near the Laurentian margin. The Grampian orogeny is also now demonstrably synchronous with the Taconian orogeny of the northern Appalachians. In Middle Ordovician time, the Laurentia-Iapetus plate boundary was characterized by ophiolite obduction, arc-continent collision, and associated short-lived subduction beneath Laurentia from at least the New England Appalachians to the Irish Caledonides.