On the 3rd May 1921, the island of Ireland was partitioned into the independent Irish Free State in the south (now the Republic of Ireland) and Northern Ireland, which remained part of the UK.
This partition was the result of a long, complicated struggle for Irish independence, provoked another long, complicated struggle, and was not the end goal for anyone involved.
So why did it happen?
A little context
While Ireland was under British rule, many British Protestants moved to the predominantly Catholic Ireland. The majority of them settled in the Northern county of Ulster, as the soil was more fertile there and thus more beneficial for them as landowners. This meant that most of those in the North were content with British rule, as it worked in their favour. Conversely, the majority of the South was tired of being governed from Westminster, which involved a lot of oppression of Catholics, and wanted independence.
The failure of political struggles for Home Rule (i.e. self-governance rather than complete independence) led to growing violence and eventually turned into full-out war between Ireland and Britain in 1919. The war ended in 1921 with the Anglo-Irish Treaty.
Partition
The Anglo-Irish Treaty created the Irish Free State, a compromise between Home Rule and complete independence. Crucially, however, the Treaty gave Northern Ireland the opportunity to opt out of the Free State and remain part of Britain, which they promptly did.
The groundwork for the idea of partition had been laid earlier with the 1929 Government of Ireland Act which created separate Home Rule parliaments for the North and South, but this was only ever meant to be a temporary solution.
It should be noted that partition was deeply unpopular with many. While Northern Unionists (those in Northern Ireland who wanted to remain with Britain) were happy, Northern and Southern Nationalists (who wanted independence for the whole island) were deeply angered at the partition, as were Southern Unionists who felt abandoned by Britain.
This anger was worsened when in 1925 it was decided that the border between the North and South would remain the same, leaving many border counties with Catholic majorities in the North.
Maps of Ireland and Northern Ireland
The result
Conflict between pro-Treaty and anti-Treaty groups in the Irish Free State turned into the Irish Civil War from 1922-1923, with a victory for the pro-Treaty side. However, continued discrimination against the Catholic minority in Northern Ireland would also lead to violence in the 60s with the start of The Troubles, a low-intensity war/insurgency fought by Irish Nationalists in the North with frequent retaliations from loyalist groups, that would last until the 1998 Good Friday Agreement.
Although the partition was never the end goal of anyone involved in the struggle around Irish independence, it ended up being the best compromise available. There is still a fair amount of resentment around the partition, and the issue returned to the news during the recent Brexit negotiations. Hopefully this Cheat Sheet provided a good introduction to such a complicated (and ongoing) issue.
For more resources on Ireland and the reality of Britain's colonial rule, head to our dedicated British History & Revisionism section.