In December 1918 a general election was held and Sinn Fein won 73 seats. However the Sinn Fein MPs refused to sit in the Britsh parliament. Instead they formed their own parliament called the Daill Eirann, which met in Dublin.
In January 1919 the Irish Volunteers renamed themselves the IRA the IRA began a guerrilla war when they shot two RIC men. The guerrilla war continued through 1920 and 1921. The British recruited a force of ex-soldiers called the Black and Tans to support the RIC. The Black and Tans were sent to Ireland in March 1920. They undertook reprisals against the IRA by burning buildings. In Dublin on 21 November 1921 they fired upon a crowd watching a football match killing 12 people. Shortly afterwards the Black and Tans burned part of Cork city center.
The war continued into 1921. On 25 May 1921 the IRA burned Dublin Customs House However 5 of them were killed and 80 were captured. Shortly afterwards, in July 1921, the war ended.
Meanwhile in 1920 the British government passed the Government of Ireland Act. By it there would be 2 parliaments in Ireland, one in the north and one in the south. However both parliaments would be subordinate to the British parliament. An election was held for the southern Irish parliament in May 1921. Sinn Fein won almost all the seats but their MPs refused to sit in the new parliament. Instead the Dail continued to meet.
Then in October 1921 a group of 5 men were appointed by the Dail to negotiate with the British. The British prime minister demanded that Ireland be partitioned and he threatened the delegates with war if they did not sign a treaty. Therefore they did so.
The Dail approved the treaty on 7 January 1922. However opinion split over the treaty with some people willing to accept it as a temporary measure and some people bitterly opposed. Fighting broke out between the IRA and the National Army. Michael Collins was killed in an ambush on 22 August 1922. The civil war in Ireland lasted until May 1923.