The state extends over an area of about five-sixths (70,273 km2 or 27,133 sq mi) of the island of Ireland (84,421 km2 or 32,595 sq mi), with Northern Ireland constituting the remainder. The island is bounded to the north and west by the Atlantic Ocean and to the northeast by the North Channel. To the east, the Irish Sea connects to the Atlantic Ocean via St George's Channel and the Celtic Sea to the southwest.
The western landscape mostly consists of rugged cliffs, hills and mountains. The central lowlands are extensively covered with glacial deposits of clay and sand, as well as significant areas of bogland and several lakes. The highest point is Carrauntoohil (1,038 m or 3,406 ft), located in the Macgillycuddy's Reeks mountain range in the southwest. River Shannon, which traverses the central lowlands, is the longest river in Ireland at 386 kilometres or 240 miles in length. The west coast is more rugged than the east, with numerous islands, peninsulas, headlands and bays.
Before the arrival of the first settlers in Ireland about 9,000 years ago, the land was largely covered by forests of oak, ash, elm, hazel, yew, and other native trees. The growth of blanket bog and the extensive clearing of woodland to facilitate farming are believed to be the main causes of deforestation during the following centuries. Today, about 12% of Ireland is forested, of which a significant majority is composed of mainly non-native coniferous plantations for commercial use. Ideal soil conditions, high rainfall and a mild climate give Ireland the highest growth rates for forests in Europe. Hedgerows, which are traditionally used to define land boundaries, are an important substitute for woodland habitat, providing refuge for native wild flora and a wide range of insect, bird and mammal species.
Agriculture accounts for about 64% of the total land area. This has resulted in limited land to preserve natural habitats, in particular for larger wild mammals with greater territorial requirements. The long history of agricultural production coupled with modern agricultural methods, such as pesticide and fertiliser use, has placed pressure on biodiversity.
The Atlantic Ocean and the warming influence of the Gulf Stream affect weather patterns in Ireland. Temperatures differ regionally, with central and eastern areas tending to be more extreme. However, due to a temperate oceanic climate, temperatures are seldom lower than −5 °C (23 °F) in winter or higher than 26 °C (79 °F) in summer. The highest temperature recorded in Ireland was 33.3 °C (91.9 °F) on 26 June 1887 at Kilkenny Castle in Kilkenny, while the lowest temperature recorded was −19.1 °C (−2.4 °F) at Markree Castle in Sligo. Rainfall is more prevalent during winter months and less so during the early months of summer. Southwestern areas experience the most rainfall as a result of south westerly winds, while Dublin receives the least. Sunshine duration is highest in the southeast of the country. The far north and west are two of the windiest regions in Europe, with great potential for wind energy generation. Ireland normally gets between 1100 and 1600 hours of sunshine each year, most areas averaging between 3.25 and 3.75 hours a day. The sunniest months are May and June, which average between 5 and 6.5 hours per day over most of the country. The extreme southeast gets most sunshine, averaging over 7 hours a day in early summer. December is the dullest month, with an average daily sunshine ranging from about 1 hour in the north to almost 2 hours in the extreme southeast. The sunniest summer in the 100 years from 1881 to 1980 was 1887, according to measurements made at the Phoenix Park in Dublin; 1980 was the dullest.