Ireland is an island on the western fringe of Europe between latitude 51 1/2 and 55 1/2 degrees north, and longitude 5 1/2 to 10 1/2 degrees west. Its greatest length, from Malin Head in the north to Mizen Head in the south, is 486 km and its greatest width from east to west is approximately 275 km. Since 1921 the island has been divided politically into two parts. The independent twenty-six county area, comprising 70,282 sq. km, has a population of 3,523,401 (1991). Northern Ireland, which is part of the United Kingdom and contains six of the nine counties of the ancient province of Ulster, has a population of 1,569,971 (1991). In 1973 Ireland became a member of the European Union (EU).
The two great mountain systems of Europe, north of the Alps, converge westwards to meet and mingle in Ireland. The older (Caledonian) extends from Scandinavia through Scotland to the north and west of Ireland, where it gives rise to the rugged and mountainous landscapes of Counties Donegal, Mayo and Galway. The higher mountains are of quartzite which weathers into bare, cone-shaped peaks such as Errigal (752 m) in Donegal, Croagh Patrick (765m) in Mayo and the Twelve Bens in Galway. Structures of similar age are responsible for the Wicklow and Blackstairs mountains which extend south-westwards from Dublin Bay for a distance of more than 100 km. In these, long-continued denudation of a great anticlinal structure has exposed a granite core which now forms rounded peat-covered uplands, the crests being notched in places by glacial cirques. The mountains are penetrated by deep glacially modified valleys of which the best known is Glendalough in County Wicklow.
The younger structures (Armorican) extend from central Europe through Brittany to southern Ireland, where they reappear as a series of east-west anticlinal sandstone ridges separated by limestone or shale-floored valleys. The hills rise in height westwards culminating in Carrantouhill (1041 m) in the Magillycuddy Reeks, the highest mountain in the country. The famous Upper Lake of Killarney nestles in the eastern slopes of this range. The valleys separating the western extension of these mountains have been flooded by the sea, giving rise to a number of long deep inlets.
In north-eastern Ireland basaltic lavas spread widely over the existing rocks in Eocene times and now form the bleak plateau of east Antrim. Westwards the basalt is downwarped and the resultant drift-covered lowland is occupied in part by Lough Neagh, the largest lake in Ireland.
The heart of the country is a limestone- floored lowland bounded on the south by the Armorican ridges and on the north and west by the Caledonian mountains. This lowland is open to the Irish Sea for a distance of 90 km between the Wicklow Mountains and the Carlingford peninsula, giving easy access to the country from the east. It also extends westwards to reach the Atlantic Ocean along the Shannon Estuary, in Galway Bay, in Clew Bay and again in Donegal Bay. Numerous hills break the monotony of the lowland which rises westward towards the coast in County Clare where it terminates in the cliffs of Moher, one of the finest lines of cliff scenery in Western Europe.
Much of Ireland was covered by ice during the Pleistocene period. This ice finally melted away about twelve thousand years ago, leaving behind evidence of its former presence in most of the minor physical features of the landscape. Throughout the greater part of the lowland the bedrock is hidden by glacial deposits which, in the north central part of the country, form a broad belt of small hills (drumlins). The glacial cover also modified the early drainage pattern and in places created groundwater conditions which facilitated the growth of peat bogs.
The lowland is drained by numerous slow- flowing streams, the largest of which is the River Shannon, 340 km in length. In its middle course this river broadens into a number of attractive lakes but as it approaches the sea its gradient steepens. This is the location of Ireland's earliest hydro-electric power scheme. The main rivers draining eastwards are the Lagan, which flows into Belfast Lough, the Liffey, with Dublin at its mouth, and the Slaney, which enters the sea at Wexford. In the south of Ireland the long east-west synclinal valleys are occupied by such rivers as the Suir, the Lee and the Blackwater which reach the coast by making right-angled turns to pass southwards through the sandstone ridges in narrow gorge-like valleys.