This paper presents explanations for some important geophysical features associated with terrane boundaries in the Iapetus Suture Zone in Ireland. The acquisition of data along the Irish Magnetotelluric Profile (IMP) provided the basis for a re-assessment of magnetic, gravity and seismic refraction results and further resolved sub-surface structures associated with terrace development during the Caledonian orogeny. The magnetotelluric data located two upper crustal conductors associated with the Fair Head-Clew Bay and Virginia magnetic Lineaments. One of the conductors probably extends along strike to Scotland, and its top surface may be associated with the T-reflector seen on the North East Coast (NEC) deep seismic reflection profile. The other conductor has a southeastern edge directly beneath a major throw in seismic basement, and defines the northern boundary of the Midland Valley Terrane in Ireland. The conductors and magnetic lineaments are probably due to black shales in sedimentary basins which have been thrust under the Avalonian and Laurentian continents during the closure of the Iapetus Ocean. The magnetotelluric data also suggest that the Southern Uplands magnetic lineament represents a crustal break but the Navan-Tipperary magnetic lineament is probably the consequence of a shallow, Volcanic block-sedimentary trough structure.