The island of Ireland is located in the north-west of Europe, between latitudes 51° and 56° N, and longitudes 11° and 5° W. It is separated from the neighbouring island of Great Britain by the Irish Sea and the North Channel, which has a width of 23 kilometres (14 mi) at its narrowest point. To the west is the northern Atlantic Ocean and to the south is the Celtic Sea, which lies between Ireland and Brittany, in France. Ireland has a total area of 84,421 km² (32,595 sq mi). Ireland and Great Britain, together with nearby islands, are known collectively as the British Isles. As the term British Isles is controversial in relation to Ireland, the alternate term Britain and Ireland is often used as a neutral term for the islands.

A ring of coastal mountains surround low plains at the centre of the island. The highest of these is Carrauntoohil (Irish: Corrán Tuathail) in County Kerry, which rises to 1,038 m (3,406 ft) above sea level. The most arable land lies in the province of Leinster. Western areas can be mountainous and rocky with green panoramic vistas. The River Shannon, the island's longest river at 386 km (240 mi) long, rises in County Cavan in the north west and flows 113 kilometres (70 mi) to Limerick city in the mid west.

The island's lush vegetation, a product of its mild climate and frequent rainfall, earns it the sobriquet the Emerald Isle. Overall, Ireland has a mild but changeable oceanic climate with few extremes. The climate is typically insular and is temperate avoiding the extremes in temperature of many other areas in the world at similar latitudes. This is a result of the moderating moist winds which ordinarily prevail from the South-Western Atlantic.

Precipitation falls throughout the year but is light overall, particularly in the east. The west tends to be wetter on average and prone to Atlantic storms, especially in the late autumn and winter months. These occasionally bring destructive winds and higher total rainfall to these areas, as well as sometimes snow and hail. The regions of north County Galway and east County Mayo have the highest incidents of recorded lightning annually for the island, with lightning occurring approximately five to ten days per year in these areas. Munster, in the south, records the least snow whereas Ulster, in the north, records the most.

Inland areas are warmer in summer and colder in winter. Usually around 40 days of the year are below freezing 0 °C (32 °F) at inland weather stations, compared to 10 days at coastal stations. Ireland is sometimes affected by heat waves, most recently in 1995, 2003, 2006 and 2013. In common with the rest of Europe, Ireland experienced unusually cold weather during the winter of 2009/10. Temperatures fell as low as -17.2 °C (1 °F) in County Mayo on 20 December and up to a metre (3 ft) of snow in mountainous areas.

The island consists 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.

In the southwest, around Bantry Bay and the mountains of Macgillicuddy's Reeks, is an area of substantially deformed, but only lightly metamorphosed, Devonian-aged rocks. This partial ring of "hard rock" geology is covered by a blanket of Carboniferous limestone over the centre of the country, giving rise to a comparatively fertile and lush landscape. The west-coast district of the Burren around Lisdoonvarna has well-developed karst features. Significant stratiform lead-zinc mineralisation is found in the limestones around Silvermines and Tynagh.

Hydrocarbon exploration is ongoing following the first major find at the Kinsale Head gas field off Cork in the mid-1970s. In 1999, economically significant finds of natural gas were made in the Corrib Gas Field off the County Mayo coast. This has increased activity off the west coast in parallel with the "West of Shetland" step-out development from the North Sea hydrocarbon province. The Helvick oil field, estimated to contain over 28 million barrels (4,500,000 m3) of oil, is another recent discovery.

There are three World Heritage Sites on the island: the Brú na Boinne, Skellig Michael and the Giant's Causeway. A number of other places are on the tentative list, for example the Burren, the Ceide Fields and Mount Stewart.

Some of the most visited sites in Ireland include Bunratty Castle, the Rock of Cashel, the Cliffs of Moher, Holy Cross Abbey and Blarney Castle. Historically important monastic sites include Glendalough and Clonmacnoise, which are maintained as national monuments in the Republic of Ireland.

Dublin is the most heavily touristed region and home to several of the most popular attractions such as the Guinness Storehouse and Book of Kells. The west and south west, which includes the Lakes of Killarney and the Dingle peninsula in County Kerry and Connemara and the Aran Islands in County Galway, are also popular tourist destinations.

Achill Island lies off the coast of County Mayo and is Ireland's largest island. It is a popular tourist destination for surfing and contains 5 Blue Flag beaches and Croaghaun one of the worlds highest sea cliffs. Stately homes, built during the 17th, 18th and 19th centuries in Palladian, Neoclassical and neo-Gothic styles, such as, Castle Ward, Castletown House, Bantry House, are also of interest to tourists. Some have been converted into hotels, such as Ashford Castle, Castle Leslie and Dromoland Castle.