The first known human settlement in Ireland began around 8000 BC, when hunter-gatherers arrived from Britain and continental Europe, probably via a land bridge.[1] Few archaeological traces remain of this group, but their descendants and later arrivals were responsible for major Neolithic sites such as Newgrange. Following the arrival of St Patrick and other Christian missionaries in the early to mid-fifth century, a syncretised form of Christianity subsumed the indigenous pagan religion by the year 600. Christianity has played a major role in Ireland's history, culture and internal conflict.

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 assimilate the invaders.

The coming of Cambro-Norman mercenaries under Richard de Clare, 2nd Earl of Pembroke, nicknamed Strongbow, in 1169 marked the beginning of more than 800 years of direct English involvement in Ireland. The English crown did not begin asserting full control of the island until after the English Reformation, when questions over the loyalty of Irish vassals provided the initial impetus for a series of military campaigns between 1534 and 1691. This period was also marked by an official English policy of plantation which led to the arrival of thousands of English and Scottish Protestant settlers. 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 Irish War of Independence, the southern and western twenty-six counties of Ireland seceded from the United Kingdom to become the independent Irish Free State — and after 1948, 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.