:Ireland is an island in Northern Europe in the north Atlantic Ocean. The island lies on the European continental shelf, part of the Eurasian Plate. The island's main geographical features include low central plains surrounded by coastal mountains. The highest peak is Carrauntoohil (), which is above sea level. The western coastline is rugged, with many islands, peninsulas, headlands and bays. The island is bisected by the River Shannon, which at with a estuary is the longest river in Ireland and flows south from County Cavan in Ulster to meet the Atlantic just south of Limerick. There are a number of sizeable lakes along Ireland's rivers, of which Lough Neagh is the largest.
Politically, the island consists of the Republic of Ireland, with jurisdiction over about five-sixths of the island, and Northern Ireland, a constituent country of the United Kingdom, with jurisdiction over the remaining sixth. Located west of the island of Great Britain, it lies at approximately . It has a total area of . It is separated from Great Britain by the Irish Sea and from mainland Europe by the Celtic Sea. Ireland forms the second largest landmass in the North-West European Archipelago, together with nearby islands including Great Britain and the Isle of Man, known in the United Kingdom as the British Isles.
Geological development
The geology of Ireland is diverse. Different regions contain rocks belonging to different geological periods, dating back almost 2 billion years. The oldest known Irish rock is about 1.7 billion years old and is found on Inishtrahull Island off the north coast of Inishowen and on the mainland at Annagh Head on the Mullet Peninsula. The newer formations are the drumlins and glacial valleys as a result of the last ice age, and the sinkholes and cave formations in the limestone regions of Clare.
Ireland's geological history covers everything from volcanism and tropical seas to the last glacial period. Ireland was formed in two distinct parts and slowly joined, uniting about 440 million years ago. As a result of tectonics and the effect of ice, the sea level has risen and fallen. In every area of the country the rocks which formed can be seen as a result. Finally, the impact of the glaciers shaped the landscape seen today. The variation between the two areas, along with the differences between volcanic areas and shallow seas, led to a range of soils. There are extensive bogs and free-draining brown earths. The mountains are granite, sandstone, limestone with karst areas, and basalt formations.
Most of Ireland was probably above sea level during the last 60 million years. As such its landscapes have been shaped by erosion and weathering on land. Protracted erosion also means most of the Paleogene and Neogene sediments have been eroded away or, as known in a few cases, buried by Quaternary deposits. Before the Quaternary glaciations affected Ireland the landscape had developed thick weathered regolith on the uplands and karst in the lowlands. There has been some controversy regarding the origin of the planation surfaces found in Ireland. While some have argued for an origin in marine planation others regard these surfaces as peneplains formed by weathering and fluvial erosion. Not only is their origin disputed but also their actual extent and the relative role of sea-level change and tectonics in their shaping. Most river systems in Ireland formed in the Cenozoic before the Quaternary glaciations. Rivers follow for most of their course structural features of the geology of Ireland. Marine erosion since the Miocene may have made Ireland's western coast retreat more than 100 km. Pre-Quaternary relief was more dramatic than today's glacier-smoothened landscapes.
Physical geography
Mountain ranges
Ireland consists of a mostly flat low-lying area in the Midlands, ringed by mountain ranges such as (beginning in County Kerry and working counter-clockwise) the MacGillycuddy's Reeks, Comeragh Mountains, Blackstairs Mountains, Wicklow Mountains, the Mournes, Glens of Antrim, Sperrin Mountains, Bluestack Mountains, Derryveagh Mountains, Ox Mountains, Nephinbeg Mountains and the Twelve Bens/Maumturks group. Some mountain ranges are further inland in the south of Ireland, such as the Galtee Mountains (the highest inland range), Silvermine and Slieve Bloom Mountains. The highest peak Carrauntoohil, at high, is in the MacGillycuddy's Reeks, a range of glacier-carved sandstone mountains. Only three peaks on the island are over and another 457 exceed . Ireland is sometimes known as the "Emerald Isle" because of its green landscape.