Ireland is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially named Ireland – a sovereign state covering five-sixths of the island) and Northern Ireland (part of the United Kingdom – covering the remaining sixth). It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the second-largest island of the British Isles, the third-largest in Europe, and the twentieth-largest in the world. As of 2022, the population of the entire island is just over 7 million, with 5.1 million in the Republic of Ireland and 1.9 million in Northern Ireland, ranking it the second-most populous island in Europe after Great Britain.
The geography of Ireland comprises relatively low-lying mountains surrounding a central plain, with several navigable rivers extending inland. Much of Ireland was woodland until the end of the Middle Ages. Today, woodland makes up about 10% of the island, compared with a European average of over 33%, with most of it being non-native conifer plantations. Its lush vegetation is a product of its mild climate, which is influenced by the Atlantic Ocean and thus very moderate, and winters are milder than expected for such a northerly area, although summers are cooler than those in continental Europe. Rainfall and cloud cover are abundant.
Gaelic Ireland had emerged by the 1st century AD. The island was Christianised from the 5th century onwards. During this period Ireland was divided amongst petty kings, who in turn served under the kings of the traditional provinces (Cúige; lit. 'fifth') vying for dominance and the title of High King of Ireland. Between the late 8th and early 11th centuries, Viking raids and settlement took place culminating in the Battle of Clontarf on 23 April 1014 which resulted in the end of Viking power in Ireland. Following the 12th-century Anglo-Norman invasion, England claimed sovereignty. However, English rule did not extend over the whole island until the 16th–17th century Tudor conquest, which led to colonisation by settlers from Britain. In the 1690s, a system of Protestant English rule was designed to materially disadvantage the Catholic majority and Protestant dissenters, and was extended during the 18th century. With the Acts of Union in 1801, Ireland became a part of the United Kingdom. The Great Famine of the 1840s saw the population fall by over 20%, through death and emigration. A war of independence in the early 20th century was followed by the partition of the island, leading to the creation of the Irish Free State, which became increasingly sovereign over the following decades until it declared a republic in 1948 (Republic of Ireland Act, 1948) and Northern Ireland, which remained a part of the United Kingdom. Northern Ireland saw much civil unrest from the late 1960s until the 1990s. This subsided following the Good Friday Agreement in 1998. In 1973, both the Republic of Ireland and the United Kingdom, with Northern Ireland as part of it, joined the European Economic Community. Following a referendum vote in 2016, the United Kingdom, Northern Ireland included, left the European Union (EU) in 2020. Northern Ireland was granted a limited special status and allowed to operate within the EU single market for goods without being in the European Union.
Irish culture has had an influence on other cultures, particularly in literature. Alongside mainstream Western culture, an indigenous culture exists, as expressed through Gaelic games, Irish music, Irish language, and Irish dance. The island's culture shares many features with that of Great Britain, including the English language, and sports such as association football, rugby, horse racing, golf, and boxing.
The names Ireland and Éire both derive from Old Irish Ériu, which in turn comes from Proto-Celtic *ɸīweriyū meaning 'fertile soil'. The Classical Latin name for the island of Ireland, Ivernia, also comes from this same root, and it remains a popular, poetic synonym for the island (usually respelt Hibernia, under the influence of hibernum, the Latin word for 'winter').
During the last glacial period, much of Ireland was periodically covered by ice sheets. The relative sea level was less than 50 m lower, resulting in an ice bridge rather than a land bridge forming between Ireland and Great Britain. By 14,000 BC this ice bridge existed only between Northern Ireland and Scotland and by 12,000 BC Ireland was completely separated from Great Britain. Later, around 6,100 BC, Great Britain became separated from continental Europe. Until recently, the earliest evidence of human activity in Ireland was dated at 12,500 years ago, demonstrated by a butchered bear bone found in a cave in County Clare. In 2021, a reindeer bone with chop marks, which had been found in Castlepook Cave near Doneraile, County Cork in the early twentieth century during Richard J. Ussher's excavations, was dated to 33,000 years ago by Dr Ruth Carden, a paleozoologist from the School of Archaeology in University College Dublin, pushing back the earliest signs of human activity by 20,000 years.
By about 8,000 BC, more sustained occupation of the island has been shown, with evidence for Mesolithic communities around the island.
Some time before 4,000 BC, Neolithic settlers introduced cereal cultivars, domesticated animals such as cattle and sheep, built large timber buildings, and stone monuments. The earliest evidence for farming in Ireland or Great Britain is from Ferriter's Cove, County Kerry, where a flint knife, cattle bones and a sheep's tooth were carbon-dated to c. 4,350 BC. Field systems were developed in different parts of Ireland, including at the Céide Fields, that has been preserved beneath a blanket of peat in present-day Tyrawley in northwest Mayo. An extensive field system, arguably the oldest in the world, consisted of small divisions separated by dry-stone walls. The fields were farmed for several centuries between 3,500 BC and 3,000 BC. Wheat and barley were the principal crops.
Ancient DNA evidence indicates that the arrival of Neolithic farmers was accompanied by a major genetic shift, with incoming farming populations largely replacing the earlier Mesolithic hunter-gatherer ancestry. The subsequent arrival of Bell Beaker-associated populations during the third millennium BC brought another significant genetic shift, including the introduction of Steppe-related ancestry, although the Early Bronze Age population also retained ancestry from earlier Neolithic populations.
The Bronze Age began around 2,500 BC, with technology changing people's everyday lives during this period through innovations such as the wheel, harnessing oxen, weaving textiles, brewing alcohol and metalworking, which produced new weapons and tools, along with fine gold decoration and jewellery, such as brooches and torcs.
How and when the island became Celtic has been debated for close to a century, with the migrations of the Celts being one of the more enduring themes of archaeological and linguistic studies. The most recent genetic research strongly associates the spread of Indo-European languages (including Celtic) through Western Europe with a people bringing a composite Bell Beaker culture, with its arrival in Britain and Ireland dated to around the middle of the third millennium BC. According to John T. Koch and others, Ireland in the Late Bronze Age was part of a maritime trading culture called the Atlantic Bronze Age that also included Britain, western France and Iberia, and that this is where Celtic languages developed. This contrasts with the traditional view that their origin lies in mainland Europe with the Hallstatt culture.
The long-standing traditional view is that the Celtic language, Ogham script and culture were brought to Ireland by waves of invading or migrating Celts from mainland Europe. This theory draws on the Lebor Gabála Érenn, a medieval Christian pseudo-history of Ireland, along with the presence of Celtic culture, language and artefacts found in Ireland such as Celtic bronze spears, shields, torcs and other finely crafted Celtic associated possessions. The theory holds that there were four separate Celtic invasions of Ireland. The Priteni were said to be the first, followed by the Belgae from northern Gaul and Britain. Later, Laighin tribes from Armorica were said to have invaded Ireland and Britain more or less simultaneously. Lastly, the Milesians (Gaels) were said to have reached Ireland from either northern Iberia or southern Gaul. It was claimed that a second wave named the Euerni, belonging to the Belgae people of northern Gaul, began arriving about the 6th century BC. They were said to have given their name to the island.