From 1919 – 1921 the Irish Republican Army ( IRA ) was organised as a guerilla army , led by Richard Mulcahy and with Michael Collins as Director of Intelligence and fought against British forces . During the Anglo-Irish War ( or War of Independence ) , the British sent paramilitary police , the " Black and Tans " and the Auxiliary Division , to help the British Army and Royal Irish Constabulary . These groups committed atrocities which included killing captured POWs and any Irish civilians they viewed as being sympathetic to the IRA . Among the most infamous of their actions were the burning of half the city of Cork in 1920 and the Bloody Sunday massacre of 1920 . These atrocities , together with the popularity of the republican ideal , and British repression of republican political expression , led to widespread support across Ireland for the Irish rebels
Bloody Sunday
Bloody Sunday (1920)
Irish republicanism