The island of Ireland historically consists of 32 counties, of which six, collectively known as Northern Ireland, have remained as part of the United Kingdom since the rest of Ireland gained self government in 1922. The name "Ireland" applies to the island as a whole, but in English is also the official name of the independent state (ie the 26 counties which are not part of the United Kingdom), since 1921.
Celtic tribes settled on the island in the 4th century BC. Invasions by Norsemen that began in the late 8th century were finally ended when King Brian Boru defeated the Danes in 1014. Norman invasions began in the early 12th century and set in place Ireland's uneasy position within England's sphere of influence. The Act of Union of 1800 - in which Catholics, 90% of the Irish population, were excluded from Parliament - saw Ireland joining the United Kingdom. In the latter half of the 19th century and early 20th century the subject of Irish home rule was a major debate within the British parliament. After several failed attempts, a Home Rule bill finally passed through parliament in 1914 though the start of the first world war saw its indefinite postponement due to heavily armed unionist opposition. A failed rebellion on Easter Monday in 1916, (after which 15 of the surrendered leaders were shot by firing squad and 1 hanged) showed a hint of things to come with years of war to follow, beginning with the Irish War of Independence (1919-1921) and continuing with the Irish Civil War (1922-1923).
Eventually a somewhat stable situation emerged with the self government of 26 of Ireland's counties known as the Irish Free State; the remaining six, located in the north of the country comprising two-thirds of the ancient province of Ulster, remained part of the United Kingdom — a status that has continued to the present day. In 1949 the Irish Free State became "Ireland" (a.k.a. the Republic of Ireland) and withdrew from the British Commonwealth of Nations.
Ireland's history post-partition has been marked with violence, a period known as "The Troubles", generally regarded as beginning in the late 1960s, which saw large scale confrontation between opposing paramilitary groups seeking to either keep Northern Ireland as part of the United Kingdom or bring it into Ireland as well as with the security forces. The Troubles saw many ups and downs in intensity of fighting and on many occasions they even spread to terrorist attacks in Britain and continental Europe. Both the government of the UK and Ireland were opposed to all terrorist groups. A peace settlement known as the Good Friday Agreement was finally approved in 1998 and is currently being implemented. All signs point to this agreement holding steady.
Though a relatively poor country for much of the 20th century, Ireland joined the European Community in 1973 (at the same time as the United Kingdom). Between the mid 1990s and late 2000s, Ireland saw massive economic boom (called 'The Celtic Tiger'), becoming one of the richest countries in Europe. However, the global banking crisis and subsequent recession have hit Ireland hard, and high levels of unemployment proliferated for the period 2009 - 2013, before a gradual ecomonic recovery.