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 independence in 1922. The name "Ireland" applies to the island as a whole, but in English is also the official name of the independent state (i.e., the 26 counties which are not part of the United Kingdom), since 1937.

Celtic tribes settled on the island in the 4th century B.C. 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. A failed rebellion on Easter Monday in 1916 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).