Ireland is an island in Northern Europe, in the north Atlantic Ocean. The island measures about 480 km (300 mi) north-south, and 275 km (171 mi) east-west, with a total area of 84,421 km² (32,595 sq mi). It lies about 53°N 8°W / 53°N 8°W / 53; -8, near the western edge of the European continental shelf, part of the Eurasian Plate. Ireland is separated from the island of Great Britain by the Irish Sea to the east, and from mainland Europe by the Celtic Sea to the south. It is bounded to the north by the North Channel and to the south by St George's Channel. Ireland is the second-largest landmass in the British Isles, after Great Britain and before Lewis and Harris.

Its main geographical features include low central plains surrounded by coastal mountains. The highest peak is Carrauntoohil (Irish: Corrán Tuathail), which is 1,039 metres (3,409 ft) above sea level. The western coastline is rugged, with many islands, peninsulas, headlands and bays, while the southern and northern coasts have a smaller number of substantial sea inlets, such as Lough Foyle and Cork Harbour; no part of the land is more than around 110 km (68 mi) from the sea. The island is almost bisected by the River Shannon, which at 360.5 km (224 mi) with a 102.1 km (63 mi) estuary is the longest river in Ireland and flows south from County Cavan in the province of Ulster to form the boundary between Connacht and Leinster, and later Munster, and meet the Atlantic just south and west of Limerick. Lough Neagh is the largest of several sizeable lakes along Ireland's rivers.

The island has a temperate oceanic climate, mild and humid, and is warmer than other landmasses at the same latitude thanks to the winds on the Atlantic Ocean, ocean currents, and circulations. The island is one of the least forested areas in Europe, though forestation is expanding, but has a strong agricultural sector. It has a limited range of mineral resources, and has only had two major gas finds, and none in the oil sector. Hydroelectric energy is used, and wind farms are growing; neither solar nor tidal energy is much exploited.

Politically, the island consists of the Republic of Ireland, with jurisdiction over about five-sixths of the island, and Northern Ireland, a constituent part of the United Kingdom, with jurisdiction over the remaining sixth. The island has traditionally been divided into four provinces, each of which is divided into counties. Of the 32 counties in total, 26 are in the Republic of Ireland, with the remaining 6 in Northern Ireland.

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 includes a wide range of elements, from volcanism and tropical seas to the last glacial period. Ireland was formed in two distinct parts, which 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.

Ireland consists of a mostly flat low-lying area in what are known as the Midlands. It is ringed by mountain ranges such as—beginning in County Cork and working clockwise—the Boggeragh Mountains, Derrynasaggart Mountains, Caha Mountains, MacGillycuddy's Reeks, Slieve Mish Mountains, Mullaghareirk Mountains, the Twelve Bens/Maumturks group, Nephinbeg Mountains, Ox Mountains, Bluestack Mountains, Derryveagh Mountains, Sperrin Mountains, the Mournes, Wicklow Mountains, Blackstairs Mountains, Comeragh Mountains and Knockmealdown Mountains. 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. There is an area of raised land near the northeastern coast, the Antrim Plateau, which contains the Glens of Antrim; this area is sometimes called the Mountains of Antrim. The highest peak Carrauntoohil, at 1,038.6 m (3,407 ft) high, is in the MacGillycuddy's Reeks, a range of glacier-carved sandstone mountains. Only three peaks on the island are over 1,000 m (3,300 ft) and another 457 exceed 500 m (1,600 ft). There are a number of walking trails in the mountains, with the longest being that through the Wicklow Mountains. In the Mourne Mountains, with multiple walking trails, a wall was built between the 13 major peaks, the 36-kilometre-long (22 mi) Mourne Wall.