Northern Ireland is made up of six counties of the Irish province of Ulster. It was formed by the Government of Ireland Act of 1920, when the whole island was still under British rule. The reason for the partition was that Home Rule for Ireland was to be restored, but the predominantly protestant population of the Northeastern counties wanted to remain under Direct Rule from Westminster. The effect of the Act was to create a devolved parliament for the six counties, which was to remain in place until the 1970s, when it was fatally destabilised by the Troubles.
However, the population of the rest of Ireland had come to view Home Rule as inadequate, and swept Sinn Féin to power in the 1918 General Election. The Sinn Féin MPs seceded from the Westminster parliament and formed a parliament in Dublin (Dáil Éireann), sparking off the Irish War of Independence. The treaty which settled this conflict contained as one of its terms the retention of partition, subject to a review of the border by an independent commission.
The supporters of the treaty on the Irish side assumed that the border commission would create a northern state so small that assimilation into an all-Ireland state would be inevitable. However, the commission's report was never released, as it was leaked that they would be recommending no significant changes to the border, and Northern Ireland remained intact.
After the Irish Civil War, the leaders of the southern political entity got on with the business of establishing it as an independent state, and more or less left Northern Ireland to its own devices. Some die-hard republicans continued to fight against partition, and the 1937 Constitution of Ireland included an aspiration to unity, but for the most part southerners had enough to occupy them.
Although the majority of the population of Northern Ireland were strongly in favour of the Union with Britain, however, the province contained a significant minority (mostly Catholic), who wanted to be part of an independent Ireland. Northern Ireland then, was a state (for this period it was ruled by a local parliament) seriously divided between predominantly protestant Unionists and predominantly catholic Nationalists. The Unionists had the upper hand, being in the majority, but on the island as a whole, however, they were massively outnumbered. A seige mentality caused their leaders to extend their local advantage through discrimination against Nationalists and catholics, gerrymandering of political constituencies, and anti-democratic electoral rules, which only extended the vote to property-owners.
This situation simmered away for decades, until the Civil Rights movement in the 1960s brought it to a head. Catholics marched in Derry and Belfast demanding equal rights in issues such as the allocation of public housing. Widespread public disorder ensued, with catholic families being burned out of their homes in areas of mixed population. This, coupled with the police (including the now disbanded B Specials) coming down hard on the protesters, led to the resurgance of the largely dormant IRA, and Loyalist organisations such as the UVF, and the start of "The Troubles".
Since then, Northern Ireland has been more or less defined in the world's eyes by this devastating and brutal conflict, although normal life has continued for most, especially those wealthy enough to avoid living in the trouble spots. The IRA's ceasefire and the Good Friday Agreement lead to hope that the conflict can be put to rest, although the peace is uneasy at best. Nonetheless, there is enormous hope that the people of Northern Ireland can put the past atrocities and inequities behind them and enjoy peace in what is, after all, one of the most beautiful parts of the world.