http://en.wikipedia.org/wiki/History_of_Ireland

The History of Ireland began around 8000 BC, when the island's first human inhabitants arrived from Britain and continental Europe, possibly via a land bridge. Few archaeological traces of these earliest hunter-gatherers remain, 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 syncretized form of Christianity subsumed the indigenous pagan religion by A.D. 600. Christianity has played a major role in Ireland's 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 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 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 Irish 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.