Ireland's geography: a rimmed plain that cannot drain

Why Ireland's mountains stand on the coast

The island was assembled from two separate pieces of crust that joined about 440 million years ago, and the seam still shows in what the rocks are. Around Galway and Donegal in the west there is a metamorphic and igneous complex of Caledonide affinity, closely comparable to the Scottish Highlands. Across south-east Ulster and running south-west to Longford and south to Navan lies a province of Ordovician and Silurian rock. Along the Wexford coastline granite has intruded into more Ordovician and Silurian rock. In the south-west, around Bantry Bay and MacGillycuddy's Reeks, the rock is deformed, lightly metamorphosed and Devonian in age. The oldest rock known on the island, about 1.7 billion years old, is on Inishtrahull off Inishowen and at Annagh Head on the Mullet Peninsula.

What those provinces have in common is position. They form a partial ring of hard rock around the outside of the island, and the middle of that ring is covered by a blanket of Carboniferous limestone. Hard rock resists erosion and limestone does not, so the ranges ended up on the rim and the centre ended up a plain.

The list of ranges reads as a circuit. Beginning in County Cork and working clockwise it runs through the Boggeragh, Derrynasaggart and Caha mountains, MacGillycuddy's Reeks, the Twelve Bens and Maumturks, the Nephinbeg, Ox, Bluestack and Derryveagh ranges, the Mournes, and the Wicklow, Blackstairs and Comeragh mountains back round to the south coast. Only the Galtee, Silvermine and Slieve Bloom mountains sit inland.

Nothing on the island is high. Carrauntoohil, in MacGillycuddy's Reeks in County Kerry, reaches 1,039 metres, and it is one of only three peaks on the island above 1,000 metres. Another 457 exceed 500 metres. The Reeks are glacier-carved sandstone, and most of Ireland has stood above sea level for the last 60 million years, long enough for weathering and rivers to take the drama out of a relief the ice then smoothed further. Marine erosion since the Miocene may have cut the western coast back by more than 100 kilometres.

The rim is also close at hand. No part of the land is more than about 110 kilometres from the sea.

How the Shannon crosses the country without falling

Two kinds of river run in Ireland and the difference between them is entirely a matter of which side of the rim they start on. Rivers rising on the seaward side of the coastal mountains are short and fast. Rivers rising inside the rim move slowly, wander through marshes and lakes, and reach the sea a long way from their sources, often dropping over waterfalls and rapids only at the very end.

The River Shannon is the extreme case. It rises in County Cavan, crosses 11 counties and runs 360.5 kilometres before an estuary of a further 102.1 kilometres carries it to the Atlantic below Limerick. Its basin covers 16,865 square kilometres, a fifth of the island. Three times on the way the river stops being a river and becomes a lake: Lough Allen, Lough Ree and Lough Derg. The flow across the central lowland is sluggish, which is what a plain with almost no gradient produces.

The water that is not moving accounts for a large share of the total. Waterbodies make up about 2% of the land area of the island. Inside the state there are over 74,000 kilometres of rivers and streams, more than 125,000 hectares of lake and over 3,000 hectares of reservoir, and none of it is spread evenly. Cork, Donegal and Mayo hold over 30% of the watercourse length. Mayo and Galway hold over 40% of the lake area. Almost the entire reservoir area sits in two counties, two-thirds of it in Wicklow and a third in Cork. Lough Corrib, at 176 square kilometres, is the largest lake inside the state.

Slow water is a problem for farming and the state has treated it as one. Arterial drainage works have improved the flow in the rivers and lowered lake levels, which prevents flooding and frees land for cultivation, and state-aided farm drainage schemes were designed to bring wasteland and marginal ground into production.

What the limestone does under the plain

The limestone under the middle of the island does two things at once. It is porous, so an underground drainage system has developed inside it that feeds the surface network of rivers and lakes from below. It also weathers into soil that grows grass.

Most Irish soils come from drift, the ice-scoured waste that was frozen to the base of the advancing glaciers. Quartzites, some granites and shales weather into unproductive soils, but in many places those have been overlaid by drift that is mostly limestone-bearing and is farmed with considerable success. The bare limestone country still visible in the west is the measure of what that cover was worth: where the ice left none, there is rock.

The Burren, around Lisdoonvarna in County Clare, is where the rock is on the surface. Its limestone formed when a tropical sea flooded the south of the island during the Carboniferous and coral built up on the floor; the sandstone and shale laid over the top afterwards have since gone from here. The rock at the base of the Cliffs of Moher, further north on the same coast, dates to about 320 million years ago. The Burren is a karst landform and one of the few places in the country where limestone can be seen above ground.

The same limestone carries the country's metals. Stratiform lead and zinc mineralisation occurs in it around Silvermines and Tynagh, and one zinc and lead mine is in operation at Tara. Gold, silver, gypsum, talc, calcite, dolomite, roofing slate, limestone aggregate, building stone, sand and gravel are the other deposits with actual or potential commercial value.

How the bogs formed and what happened to them

Bog covers about 12,000 square kilometres of the island in two forms that were made in completely different ways.

Why the midlands hold the raised bogs

Raised bogs are the midland type and the Shannon basin is where they are most common. They began as depressions left behind by the last ice age that filled with water and became lakes. Debris from reeds settled on the bottom, the layer thickened until it choked the lake, and in time it rose above the level of the ground around it, which is what the name records.

Blanket bogs were made by people. Neolithic farmers cleared trees for farming; the cleared ground fell out of use, the soil leached and turned acid, heather and rushes took it over, and the debris from those plants accumulated as peat under a climate wet enough to keep the process running. County Mayo holds the widest expanse of Atlantic blanket bog on the island. The two types occur in combinations that are found nowhere else in Europe.

What turf cutting took out of them

Peat has been cut for fuel since the 17th century, and cut peat is called turf. Machine cutting arrived in the 1940s and made large-scale harvesting possible; in the state this became the work of a semi-state company, Bord na Móna. Commercially extracted turf went into peat briquettes for domestic heating and milled peat for electricity generation, and peat has more recently been combined with biomass for dual-firing generation.

Raised bog yields a higher-grade fuel than blanket bog and was mined harder for it. The National Parks and Wildlife Service ran a series of national bog surveys from the 1980s to the mid-2000s, and midland raised bog is the type that came out of them worst: less than 1% of it is in a natural state and actively forming peat. Under the 1992 European Habitats Directive the state is required to protect active raised bog, and 139 raised bogs have since been designated inside 53 Raised Bog Special Areas of Conservation, with a further 75 designated as Natural Heritage Areas under the Wildlife (Amendment) Act 2000.

Habitat is one of several things at stake in that designation. Intact bogs absorb, hold and slowly release water, which purifies it and reduces flooding, and while they are forming they move carbon dioxide out of the air into storage that lasts thousands of years. They also keep what was put into them or built across them, including trackways and bog bodies.

Why the Atlantic decides the Irish weather

The climate is classified Cfb, temperate oceanic, and the controlling feature is the ocean on the west side. The warm North Atlantic Drift holds up sea temperatures, the maritime influence is strongest on the Atlantic coasts and weakens inland, and the hills and mountains, most of them near the coast, shelter the interior from the strongest winds. Winters are cool and windy. Summers, when the depression track has moved north and the depressions are shallower, are mild and less windy.

The mechanism is the polar front, the zone where warm moist air moving north meets colder, denser, drier air moving south. Depressions form on waves in that front, cross the North Atlantic and usually pass to the north-west of Ireland, which gives the sequence that defines the weather: rain ahead of the centre, then brighter, colder air with showers behind it. An examination of synoptic charts over ten years found an average of 170 fronts a year reaching the south-west of the island, in a figure published in 1975.

The temperature numbers are unusually flat. The mean temperature over 1990 to 2020 was 9.8 °C. January and February are the coldest months, with mean daily air temperatures of 5.3 to 5.5 °C. July and August are the warmest, at 15 to 15.2 °C, with mean daily maxima of 18.9 to 19.1 °C, lower near the coast and higher inland. May and June are the sunniest months, with five to seven hours of sunshine a day. Dublin runs about 9 °C warmer in winter than St John's in Newfoundland, which sits at a similar latitude on the far side of the same ocean.

Rain is where the west and the east part company. Valentia Island, off County Kerry, takes over twice the annual rainfall Dublin does, 1,557 millimetres against 714. Across the country the range runs from about 760 millimetres a year in the east to more than 2,533 millimetres in the western areas that take the Atlantic cloud head-on. February to June are the months of least rainfall, when continental and Greenland anticyclones block the depressions out.

Extremes are shallow and infrequent. Inland stations record about 40 days a year below freezing against 10 at coastal stations. Snow falls most often in January and February and usually lies for a day or two, though the more notable falls have lain for 10 to 12 days. Atlantic depressions between December and February can bring winds of up to 160 kilometres an hour to western coastal counties. The highest temperature recorded on the island, 33.3 °C, was measured at Kilkenny Castle on 26 June 1887.

What grows in a country with almost no forest

Ireland was once covered in temperate rainforest, as much of Atlantic Europe was. Clearing began with the first farmers in the Neolithic and accelerated after the Plantations, and by the end of the 19th century woodland was down to about 1% of the land area. Total tree cover in the state stood at 11% of land area in 2017 and native forest at 2% in 2018, against an average across European countries of over 33%. Most of what has gone in since is non-native conifer, and state-sponsored replanting has favoured fast-growing sitka spruce. About 389,356 hectares are in state ownership, mainly through the forestry company Coillte. The surviving native woodland is scattered, and the best-known remnant is in Killarney National Park.

What replaced the trees is grass. Steady precipitation and an equable temperature suit grassland, and grassland is what the country's livestock run on. Hedgerows, planted to mark boundaries, do a good deal of the work woodland would otherwise do, carrying native flora and a range of insects, birds and mammals across a countryside given over to fields. Agriculture drives the land use pattern and limits the ground available to larger wild mammals, and about 32% of Ireland's greenhouse gas emissions correlate with it. Grazing by deer and sheep on uncultivated ground is one of the main things preventing forest from regenerating on its own.

The species list is short for a reason older than farming. Great Britain kept a land bridge to mainland Europe; Ireland had only an ice bridge, and it ended around 14,000 years ago, which cut the migration off early. There are 55 mammal species on the island and only 26 land mammals counted as native. About 400 bird species have been recorded, many of them migratory. There are no snakes, and the common lizard is the only reptile native to the island. The Irish elk, the great auk, the brown bear and the wolf are gone; the golden eagle was reintroduced after decades of absence.

Where the coastline breaks into islands and peninsulas

The coastline of the island runs to 7,524 kilometres, a long edge for a landmass measuring about 480 kilometres north to south and 275 kilometres east to west.

West and east are different coasts. The western side is broken into islands, peninsulas, headlands and bays. The southern and northern coasts have fewer but larger inlets, among them Cork Harbour at the mouth of the Lee. On the east there is nothing of size between Dublin Bay and Wexford Harbour at the mouth of the Slaney.

The south-west is where the pattern shows clearest, because the ranges and the sea interlock there. The Dingle, Iveragh and Beara peninsulas run out into the Atlantic with Dunmanus Bay, the Kenmare estuary and Dingle Bay between them. North of those is the Shannon Estuary, then Galway Bay between north Clare and Galway, then Clew Bay in Mayo below Achill Island, then Broadhaven, Blacksod and Killala bays, then Donegal Bay between Donegal and Sligo.

Achill is the largest of the offshore islands, is inhabited and is permanently joined to the mainland by a bridge. Its Atlantic coast carries Croaghaun, which rises 688 metres; the Slieve League cliffs in Donegal reach 601 metres. The Aran Islands off southern Connacht hold an Irish-speaking Gaeltacht. Valentia Island, off Iveragh, is settled and bridged at its south-eastern end.

The named points fix the extremes. Malin Head in Donegal is the most northerly point on the island and Mizen Head close to the southernmost, which is why anything applying to the island as a whole is described as running from Malin to Mizen. Carnsore Point is the south-easternmost, the Fastnet Rock the most southerly. Inishtrahull, off Malin Head, is the most northerly undisputed land feature. Rockall lies further north and is claimed by the United Kingdom, Ireland, Denmark on behalf of the Faroe Islands, and Iceland; this catalogue records the claim and takes no position on it.

The shelf under all of this extends far to the west and south-west. Seabed claims made by the larger of the island's two jurisdictions exceed 880,000 square kilometres, more than ten times its land area. Along the landward edge of the same coast runs the Wild Atlantic Way, a driving route of 2,500 kilometres.

How the land decided where people settled

The state holds 69,797 square kilometres of the island's 84,421, about 83%, and 26 of its 32 counties. The other six make up Northern Ireland, and the boundary between them is the state's only land border, shared with the United Kingdom.

Administration follows the counties more closely than it follows the terrain. Twenty-three counties are local government areas; Dublin, Cork and Galway have city councils administered separately from the counties of the same name, and the remainder of County Dublin is divided into Dún Laoghaire-Rathdown, Fingal and South Dublin, which produces 31 local authorities in all. Limerick and Waterford were merged with their county councils in 2014. County Tipperary had two ridings, established in 1838, renamed in 2001 and amalgamated in 2014. When Dáil constituencies are redrawn, the Electoral Commission is required to avoid breaching county boundaries as far as that is practicable.

Population sits east and south. The census of April 2022 recorded 5,149,139 people in the state, concentrated in and around Dublin and Cork. The most arable land is in Leinster, which is also where most of those people live, and the two facts are the same fact.

The protected land went the other way, out onto the rim. Every one of the state's national parks is in the coastal ranges, on the western karst or at sea, and none is in the central plain where the farming and the population are.

The same geography carries the electricity. The first Shannon hydroelectric scheme was built in 1929, within seven years of independence, with a capacity of 86 MW and annual generation of 332 GWh; Turlough Hill, a pumped-storage station built into a mountain, added 292 MW when it was completed in 1974. Wind arrived much later and arrived onshore: the state had more than 300 wind farms as of 2022, while the single offshore farm at Arklow Bank produces 0.6% of the national wind total.