Pre-Caledonian basement rocks have been recognized structurally beneath the Dalradian cover in NW County Mayo for over 20 years and have more recently been subdivided into an older Annagh Division, dominantly of orthogneisses which contain granitic components of Grenvillian age, and a younger Inishkea Division, dominantly of semi-pelitic and psammitic lithologies. Part of this division was originally interpreted as having been derived from probable Annagh Division rocks by structural and metamorphic reconstitution, but has more recently been considered as an entirely meta-sedimentary sequence older than the Dalradian cover. A structural and metamorphic history older than that of the Dalradian cover has been assumed, but is difficult to substantiate. Re-examination of critical localities confirm that the Annagh Division is older than the Inishkea Division, but also demonstrates that the exposed contact of the latter with the cover is stratigraphic. The Inishkea Division includes lithostratigraphic units that occupy different parts of the Dalradian cover successions.