Abstract The predominantly quartzofeldspathic gneisses of the Mullet peninsula and adjoining mainland south of Belmullet, Co. Mayo, Ireland, lie structurally below the Dalradian, with which all contacts are tectonic (Fig. 20). They were correlated by Hull (1881) with the Laurentian basement of Canada, whereas Kilroe (1907) and Trendall & Elwell (1963) considered them to be Caledonian migmatites. Sutton (1969), Sutton & Max (1969) and Max (1970) recognized the gneisses as a pre-Caledonian metamorphic complex within the Caledonides. Sutton (1972) introduced the name Annagh Gneiss Complex to describe the gneisses of the Mullet peninsula and tentatively correlated the rocks with the Caledonized Lewisian inliers of NW Scotland. This view was accepted by most workers (e.g. Watson 1975) until Rb-Sr isotopic data cast doubt on the existence of Archaean crust within the complex (van Breemen et al. 1976). On the basis of Sm-Nd and U-Pb zircon isotopic data, recent workers have correlated the Annagh Gneiss Complex with early Proterozoic rocks in Islay, SW Scotland, and Inishtrahull, Co. Donegal (Menuge & Daly 1990; Daly et al. 1991; Dickin & Bowes 1991).