The Cuilcagh Dyke in Co. Fermanagh forms part of the NW-SE trending Donegal-Kingscourt dyke swarm. Dykes in this swarm, along with others in the Irish Tertiary Igneous Province, are associated with negative magnetic anomalies. Segments of the Cuilcagh Dyke have these negative magnetic anomalies but some parts have been found to be associated with positive magnetic anomalies. Susceptibility measurements also show significant variations along the dyke. Palaeo-magnetic investigations suggest that parts of the dyke formed during a period of reverse polarity, similar to the rest of the Irish Tertiary Igneous Province, and parts formed during normal polarity conditions. The Cuilcagh Dyke in unique in Ireland in that it is the only known dyke to possess both normal (positive) and reverse (negative) polarity magnetic characteristics.