After a lull in violence in the north over the new year, killings there intensified again in the spring of 1921. The northern IRA units came under pressure from the leadership in Dublin to step up attacks in line with the rest of the country. Predictably, this unleashed loyalist reprisals against Catholics. For example, in April 1921, the IRA in Belfast shot dead two Auxiliaries in Donegal Place in Belfast city centre. The same night, two Catholics were killed on the Falls Road. On 10 July 1921 the IRA ambushed British forces in Raglan street in Belfast. In the following week, sixteen Catholics were killed and 216 Catholic homes burned in reprisal - events known as Belfast's Bloody Sunday. Killings on the loyalist side were largely carried out by the Ulster Volunteer Force , allegedly with the aid of the RIC and especially the auxiliary police force, the Ulster Special Constabulary or "B-Specials". The Special Constabulary , was largely recruited from Ulster Volunteer Force and Orange Lodges and, in the words of historian Michael Hopkinson, "amounted to an officially approved UVF". In May James Craig came to Dublin to meet the British Lord Lieutenant of Ireland, Lord FitzAlan, and was smuggled by the IRA through Dublin to meet Eamon de Valera. The two leaders discussed the possibility of a truce in Ulster and an amnesty for prisoners. Craig proposed a compromise settlement based on the Government of Ireland Act, 1920, with limited independence for the South and autonomy for the North within a Home Rule context. However, the talks came to nothing and violence in the north continued.

Answer this question based on the article: How many Catholics were murdered by the IRA IN 1921?
18