Why did Partition occur in Ireland in 1922?

This question should be answered with reference to a number of factors, which should be laid out and clearly signposted in the introduction. The key factors to discuss are:

1. Protestant Ulster Unionism vs Catholic Nationalism and the two different Irish identities. The answer should point to the largely irreconciliable nature of these two identities, although may show balance by highlighting the existence of Anglo-Irish nationalists such as W.B. Yeats or Constance Markiewicz (who later converted to Catholicism).

2. In relation to the above, the answer should include examples of sectarian violence towards minorities, whether it be towards Protestants in the South or Catholics in the North. Fear of being a minority on both sides was a causal factor for this violence and answers could highlight Partition as a means to solve this.

3. Answers must include the importance of previous negotiations with Britain and the third Home Rule Bill. Unionists had already extracted certain assurances that Ulster, or at least, certain counties of Ulster, would remain British if Home Rule was passed (something they fought vehemently against). Lloyd George and the British government also could not ignore the sacrifice by Ulstermen in the First World War (e.g. the 2,000 men lost on the first day of the Somme alone). British public opinion was unlikely to stand for all out war against Ireland to keep it under British control but nor was it likely to support the abandonment of Ulster Unionists. Partition was therefore a compromise that a war-weary population could accept. 

The conclusion should highlight that Partition was a compromise, undesirable on both sides to an extent but in light of the differing identies and political conditions, pretty much unavoidable.