The below was supplemental to a writeup by dunne, now gone, about the partition of Ireland. In it, he made the claim that partition was something cooked up by Lloyd George and friends at the negotiations following the Irish War of Independence, in order to fatally compromise the new political entity in the 26 counties. I'm leaving it stand as it still provides a fairly comprehensible discourse on partition. If anything in the below is unclear, please /msg me and I will try to fix it.
The partition of Ireland was already in effect when the Anglo-Irish Treaty negotiations began in 1921, having been instituted in the Government of Ireland Act 1920. The treaty of 1921 acknowledged partition, but this was not the main reason its adoption led to Civil War in 1922. This conflict was fought on the issue of whether the Irish people should accept a measure of independence less than that of a full Republic. Particularly divisive was the provision that representatives to the Free State's parliament would be required to swear allegience to the British monarch.
dunne: "Partition was designed to ensure the continuance of a pro-British majority in at least some part of Ireland, and thus aid in the continuation of British dominance over the island as a whole."
This theory is not implausible, given that there must have been many in the British establishment of the time who could not countenance the idea of Ireland seceding from the Empire. However, dunne doesn't mention at all the much more pressing, pragmatic reasons why Partition was preserved in the Treaty: neither party could afford to force the Unionists in the six counties to participate in an all-Ireland state.
When Home Rule for Ireland was mooted before the First World War, the Ulster Volunteers was formed, with the aim of fighting Home Rule "with their last drop of blood". They became the first group in Ireland to arm themselves and openly show their strength. The Irish Volunteers, which eventually merged with other groups to become the IRA, were formed as a direct reaction to this threat to Home Rule.
The Government of Ireland act was an attempt to diffuse this potentially explosive situation, essentially by allowing Home Rule to both parts of the island separately. However, in the wake of the executions of the leaders of the 1916 Rising, it became clear that Irish Nationalists were going to settle for nothing less than full independence, and the Irish War of Independence ensued. During this conflict, the Republicans made it clear to Britain that it could not hope to maintain order unless Ireland was given independence. Neither, however, could it hope to maintain order in the six counties unless the Unionist population were given their way and allowed to remain within the United Kingdom.
The solution of Partition, of course, was a fudge, as it created a state with a significant Irish nationalist minority, and laid the seeds for the recent conflict. However, it is clear that at the time it seemed like the way to preserve some semblence of law and order on the island.