The History of Ireland is the history of a large island in the north-west of Europe. It has been heavily influenced by the concurrent History of Britain (its larger neighbour to the east) and by Europe as a whole.

The first humans inhabited Ireland from around 7500 BC and were later responsible for major Neolithic sites such as Newgrange. Following the arrival of St. Patrick and other Christian missionaries in the mid-fifth century, a syncretized form of Christianity subsumed the indigenous pagan religion by A.D. 600. Christianity has played a major role in Ireland's subsequent history and culture.

From around 800, more than a century of Viking invasions wreaked havoc upon the monastic culture and on the island's various regional dynasties, yet both of these institutions proved strong enough to survive and assmilate the invaders.

The coming of Anglo-Norman mercenaries under Strongbow in 1169 marked the beginning of more than 800 years of direct English involvement in Ireland. The Crown of England did not gain full control until the 16th and 17th centuries, when the whole island had been subjected to numerous military campaigns in the period 1534–1691, and was colonised by English and Scottish Protestant settlers. Most of the Irish remained Roman Catholic. From this period on, sectarian conflict became a recurrent theme in Irish history.

Throughout this period, Ireland regained a form of self-governing status through the Parliament of Ireland, but power was limited to the Anglo-Irish, Anglican minority while the majority Roman Catholic population suffered severe political and economic privations. In 1801, this parliament was abolished and Ireland became an integral part of a new United Kingdom of Great Britain and Ireland under the Act of Union.

In 1922, after the War of Independence, the southern and western twenty-six counties of Ireland seceded from this United Kingdom and became the independent Irish Free State — now legally described as the "Republic of Ireland". The remainder of the island, known as "Northern Ireland", remained part of the UK. The history of Northern Ireland has been dominated by sporadic sectarian conflict between (mainly Catholic) Nationalists and (mainly Protestant) Unionists. This conflict erupted into the Troubles in the late 1960s, until an uneasy peace 30 years later.

Niall Noigiallach (died c.450/455) laid the basis for the Uí Néill dynasty's hegemony over much of western, northern and central Ireland. Politically, the former emphasis on tribal affiliation had been replaced by the 700's by that of patrilinial and dynastic background. Many formerly powerful kingdoms and peoples disappeared. Irish pirates struck all over the coast of western Britain in the same way that the Vikings would later attack Ireland. Some of these founded entirely new kingdoms in Pictland, Wales and Cornwall. The Attacotti of south Leinster even served in the Roman Legions in the mid-to-late 300's.[citation needed]

Perhaps it was some of the latter returning home as rich mercenaries, merchants, or slaves stolen from Britain or Gaul, that first brought the Christian faith to Ireland. Some early sources claim that there were missionaries active in southern Ireland long before St. Patrick. Whatever the route, and there were probably many, this new faith was to have the most profound effect on the Irish.

Tradition maintains that in AD 432, St. Patrick arrived on the island and, in the years that followed, worked to convert the Irish to Christianity. On the other hand, Palladius was sent to Ireland by the Pope in 431 as "first Bishop to the Irish believing in Christ", which demonstrates that, by whatever means, there were already Christians living in Ireland. Palladius seems to have worked purely as Bishop to Irish Christians in the Leinster and Meath kingdoms, while Patrick — who is now believed to have arrived as late as 461 — worked first and foremost as a missionary to the Pagan Irish, converting in the more remote kingdoms located in Ulster and Connacht.

The druid tradition collapsed, first in the face of the spread of the new faith, and ultimately in the aftermath of famine and plagues due to the climate changes of 535–536. Irish scholars excelled in the study of Latin learning and Christian theology in the monasteries that flourished shortly thereafter. Missionaries from Ireland to England and Continental Europe spread news of the flowering of learning, and scholars from other nations came to Irish monasteries. The excellence and isolation of these monasteries helped preserve Latin learning during the Early Middle Ages. The arts of manuscript illumination, metalworking, and sculpture flourished and produced such treasures as the Book of Kells, ornate jewelry, and the many carved stone crosses that dot the island. Sites dating to this period include clochans, ringforts and promontory forts.

Early medieval era 800–1166

The first recorded Viking raid in Irish history occurred in 795 when Vikings from Norway looted the island of Lambay, located off the Dublin coast. Early Viking raids were generally small in scale and quick. These early raids interrupted the golden age of Christian Irish culture starting the beginning of two hundred years of intermittent warfare, with waves of Viking raiders plundering monasteries and towns throughout Ireland. Most of the early raiders came from the fjords of western Norway.

By the early 840's, the Vikings began to establish settlements along the Irish coasts and to spend the winter months there. Vikings founded settlements in Limerick, Waterford, Wexford, and most famously, Dublin. Written accounts from this time (early to mid 840's) show that the Vikings were moving further inland to attack (often using rivers such as the Shannon) and then retreating to their coastal headquarters.

In 852, the Vikings Ivar Beinlaus and Olaf the White landed in Dublin Bay and established a fortress, on which the city of Dublin (from the Irish Gaelic An Dubh Linn meaning "the black pool") now stands. Olaf was the son of a Norwegian king and made himself the king of Dublin. After several generations a group of mixed Irish and Norse ethnic background arose (the so-called Gall-Gaels, Gall then being the Irish word for "foreigners" — the Norse). The descendants of Ivar Beinlaus established a long dynasty based in Dublin, and from this base succeeded in dominating much of the isle. This rule was ultimately broken by the joint efforts of Maelsechlainn II, King of Meath, and Brian Boru (c. 941–1014) at the Battle of Clontarf where Brian Boru died.

Early Ireland had an unusual government. Ireland was divided into many small kingdoms called tuaths. Each tuath's king was elected by all the free men on its territory. The tuath was thus a body of persons voluntarily united and its territorial dimension was the sum total of the landed properties of its members. About 80 to 100 tuatha coexisted at any time throughout Ireland. Above the tuaithe were larger provincial kingdoms.

Later Medieval Ireland

The Coming of the Normans 1167–1185

By the 12th century, Ireland was divided politically into a shifting hierarchy of petty kingdoms and over-kingdoms. Power was concentrated into the hands of a few regional dynasties contending against each other for control of the whole island. One of their number, the King of Leinster Diarmait Mac Murchada (anglicised as Diarmuid MacMorrough) was forcibly exiled from his kingdom by the new High King, Ruaidri mac Tairrdelbach Ua Conchobair. Fleeing to Aquitaine, Diarmait obtained permission from Henry II to use the Norman forces to regain his kingdom. The first Norman knight landed in Ireland in 1167, followed by the the main forces of Normans, Welsh and Flemings in Wexford in 1169 Within a short time Leinster was regained, Waterford and Dublin were under Diarmait's control, and he had Strongbow as a son-in-law, and named him as heir to his kingdom. This caused consternation to King Henry II of England, who feared the establishment of a rival Norman state in Ireland. Accordingly, he resolved to establish his authority.

Henry landed with a large fleet at Waterford in 1171, becoming the first King of England to set foot on Irish soil. Henry awarded his Irish territories to his younger son John with the title Dominus Hiberniae ("Lord of Ireland"). When John unexpectedly succeeded his brother as King John, the "Lordship of Ireland" fell directly under the English Crown.