Geologically the island consists of a number of provinces – in the far west around Galway and Donegal is a medium to high grade metamorphic and igneous complex of Caledonide (Scottish Highland) affinity. Across southeast Ulster and extending southwest to Longford and south to Navan is a province of Ordovician and Silurian rocks with more affinities with the Southern Uplands province of Scotland. Further south, there is an area along the Wexford coast of granite intrusives into more Ordovician and Silurian rocks with a more Welsh affinity.^[25][26]

Carrauntoohil, the highest peak in Macgillycuddy's Reeks

In the southwest, around Bantry Bay and the mountains of Macgillicuddy's Reeks, is an area of substantially deformed but only lightly metamorphosed Devonian-aged rocks.^[27]

This partial ring of "hard rock" geology is covered by a blanket of Carboniferous limestone over the centre of the country, giving rise to the comparatively fertile and famously "lush" landscape of the country. The west coast district of The Burren around Lisdoonvarna has well developed karst features.^[28] Elsewhere, significant stratiform lead-zinc mineralisation is found in the limestones (around Silvermines and Tynagh).

Hydrocarbon exploration is ongoing. The first major find was the Kinsale Head gas field off Cork/Cobh by Marathon Oil in the mid-1970s.^[29][30] More recently, in 1999, Enterprise Oil announced the discovery of the Corrib Gas Field. This has increased activity off the west coast in parallel with the "West of Shetland" step-out development from the North Sea hydrocarbon province. The Helvick oil field, estimated to contain over 28 million barrels (4,500,000 m³) of oil, is another recent discovery.^[31]