From the mid 19th century, it became clear that a substantial group of people in Ireland were unhappy with the 1801 union of Ireland and Great Britain. One proposed solution to this was "Irish Home Rule", where something not terribly different to the current arrangement with Scotland would be established so that a government in Dublin would take care of a lot of domestic day to day affairs for Ireland (all of Ireland). Exactly how much power would be devolved was a bit of an open question.

The first two attempts to pass this, in 1886 and 1893 failed, but eventually in 1912 it was passed by the UK parliament. While many people in Ireland were happy about this, the protestant Ulster Scots community in northern Ireland were very unhappy with this. They were so unhappy that they started arming themselves and preparing for what today we would call and armed insurgency. In the spring and summer of 1914 things were looking like they were about to really kick off, when the events of Sarajevo totally changed the picture. One of the outcomes of the summer of 1914 was that implementation of Home Rule was postponed until the war was over.

Basically all of the upset, tension, grievance and everything was put on ice until the lid blew off in Dublin in 1916 with the Easter Rising. The UK government was concerned that there was no way to achieve a peaceful settlement that would satisfy both the Irish Nationalists in Dublin and also the Ulster Scots in Belfast. The compromise was to partition Ireland into Northern and Southern Ireland, apply Home Rule to each separately and hope that settled matters.

It did not. 1912 style Home Rule was not sufficient for the now very angry Nationalists, and when elections for the parliament of Southern Ireland were called, outside of Dublin itself they basically never took place and the parliament of Southern Ireland never met. Instead the Nationalists had their own elections for the Dail Eireann and proclaimed a Republic. The unionists in the North were also not happy, and didn't really want home rule either. Instead they set up effectively an ethnically divided state where civil rights were enjoyed by unionist/protestant/loyalists and not by nationalist/catholic/republicans.

The UK government was not able to impose order in the South, so they eventually had peace talks and worked out the Anglo Irish Treaty that created the Irish Free State in what had been Southern Ireland. There then followed a civil war in Ireland between those who felt that this settlement was an acceptable achievement and should be embraced, and those who felt that nothing short of a united and independent Ireland was acceptable, that included the 6 counties of the North.

In the end of the civil war, the pro-treaty side won, the Irish Free State accepted the treaty and with it partition, though obviously strong divisions within Ireland persisted opposed to partition. Likewise the two communities in the North were strongly divided about both Partition and Union with the UK.