Although now largely disappeared within the 400 Ma-old Galway Granite, the syntectonic Connemara metagabbro–gneiss complex, which crystallized at 490 Ma (Arenig), is the largest intrusion into the Dalradian rocks. It is part of widespread Cambro–Ordovician volcanism and plutonic injection into continental crust in the Appalachian–Caledonide orogenic belt, being synchronous in crystallization with the widely obducted 490 Ma ophiolites within the orogeny. The peridotites and hornblende gabbros were injected, broken up, and shouldered aside by quartz diorite, granodiorite, granite and trondhjemite gneisses, and it is shown that these orthogneisses are not the in situ differentiation products of the gabbro magma. The hornblende in the metagabbros is mainly igneous but is partly metamorphic and replaces olivine (Fo75–92), orthopyroxene (En65–92), clinopyroxene (Wo48–5En50–39FS5–14) and plagioclase typically An80–92) (range An50–96) An aureole at least 10 km wide in the sillimanite zone was imposed, and cordierite-garnet-sillimanite Dalradian hornfelses were partially melted at temperatures up to 1000 oC near to the gabbros at 4.5 ± 1 kbar corresponding to c. 18 km depth. This suggests that the complex forms part of the root of a now eroded batholith of calc-alkaline affinities, the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths. Within the orthogneisses are sheets of paragneiss formed from Dalradian metasediments which have been folded up from the floor and down into sides of the magma chamber, which process could be illustrative of the means by which basic magma can intimately interact with crustal material. The 18O/16O, D/H, Nd and Sr isotope evidence, and the existence of inherited zircons in the gneisses, indicate that the gabbro and gneiss magmas were significantly contaminated with crustal, possibly Dalradian, material both before and after intrusion, with the lowest initial 87Sr/86Sr at 490 Ma being 0.708 even in samples far removed from metasediment. The metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc-alkaline geochemistry. The chemistry, igneous hornblende and highly calcic plagioclase match many arc plutons in the western USA. The syntectonic nature and the ubiquitous gneissic fabric in the quartz diorites, trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the Connemara massif, and which culminated in slicing off the largely inverted complex and carrying it southwards on the Mannin Thrust which underlies the whole of Connemara. The complex is thus a fragment of a now disrupted Cambro-Ordovician continental magmatic arc or continent-island arc environment which can be traced along the SE edge of the Laurentian continent.