The Caledonian and pre-Caledonian intrusive rocks of Ireland comprise three major groups, each widely separated in time.

1. Precambrian:
Small outlying areas of basic and granitic gneiss are located near the flanks of the Caledonide belt in the extreme north and south of Ireland. They are little-modified fragments of the older basement. On the basis of structural position and comparative lithology, they are identified with older Precambrian formations.

2. Plutonic rocks of crystalline Caledonides:
Extensive regional metamorphic tracts in the north, northwest, and west originated in what is considered to be an early Caledonian orogenic phase. It is allocated on geologic grounds to pre-Arenigian time. Emplacement of gabbroic intrusions followed by granodioritic and potash-feldspar migmatization occurred during this phase.

3. Intrusions of late Caledonian orogenic phase:
This phase is characterized by several separate periods of deformation and the intrusive suites that are associated broadly with each. A range of basic sheet intrusions is distributed widely in the lower Paleozoic graywacke-slate massifs. After intrusion of the basic sheets, late synkinematic and postkinematic granite plutons were intruded--commonly with characteristic lamprophyre associates. The granites and other intrusives extend to the crystalline Caledonides, and are between 360 and 400 m.y. old. These ages are within the range of the classic Caledonian orogeny. It has been postulated, on geologic grounds, that a few granite bodies may be as old as middle Ordovician. Distinctive, small ultrabasic intrusions are present along the Highland Boundary zone.

The structural and petrologic relations of these bodies are considered herein.