The island consist of varied geological provinces . In the far west , around County Galway and County Donegal , is a medium to high grade metamorphic and igneous complex of Caledonide affinity , similar to the Scottish Highlands . Across southeast Ulster and extending southwest to Longford and south to Navan is a province of Ordovician and Silurian rocks , with similarities to the Southern Uplands province of Scotland . Further south , along the County Wexford coastline , is an area of granite intrusives into more Ordovician and Silurian rocks , like that found in Wales .