Magnetic data acquired over a range of lithologies show the following: the Deer Park Complex of Clew Bay exhibits different magnetic characteristics east and west of Croagh Patrick. Very large magnetic signatures (4000-8000nT) occur in the west but the anomahlies are significantly lower in the east. In addition, the peak anomaly bounds the northern edge of the Deer Park Complex in the west but the southern edge to the east Lower Palaeozoic volcanics flanking the Leinster Granite yield anomalies of 300nT with a variance of 5-30nT, whereas those in the Kildare Inlier have a variance of 20-100nT and anomalies of 700nT. The background variation in magnetism for the Northern, Upper Liffey Valley and Lugnaquillia Units of the Leinster Gramnite is 2-5nT (similar to the Lower Palaeozoic sediments into which the gramnite is intruded) and structures are associated with anomalies of 30nT. The magnetic variance within the Dalradian in County Tyrone is about 5nT and peak anomalies of around 30nT have been obtained. Structures within Tertiary basalts and gabbros produce anomalies of a similar magnitude (1000-2000nT), but the variance for the basalts (300nT) is an order of magnitude greater than the variance of the gabbros. Structures in a range of hlithologies (including Carboniferous limestone) exhibit similar magnetic patterns, though the amplitude of the peak anomaly.