the Irish War for Independence was a guerrilla war mounted against the British government by Irish citizens. the revolt began during the Easter Rising of 1916, during which various factions proclaimed an independent republic. during the subsequent period of violent repression by the British Army, by its intelligence agency MI5, and by the loyalist Irish Auxilliary Division, rebel factions united under the umbrella of the Irish Republican Army (IRA), led by Michael Collins. in 1919 the IRA's political arm, Sinn Fein, under Eamon de Valera, issued a formal declaration of independence and established an elected assembly, the Dail Eireann.
the war continued until July 1921, when the Anglo-Irish Treaty ended British rule in most of Ireland, establishing the Irish Free State. however, the northeasternmost 6 of Ireland's original 32 counties, being predominantly Protestant and loyal to the Crown, opted out of the treaty, choosing to remain under British rule as Northern Ireland. the remaining 26 counties, mostly Catholic, form the present-day Irish Republic.
during the war, one of the most significant events was Bloody Sunday -- 21 November 1920. on that day, an IRA operation in Dublin led to the deaths of 12 British agents and 2 Irish sympathizers. in reprisal, that afternoon the British Army and the Auxiliary Division surrounded and attacked unarmed civilian spectators at a Gaelic football match in Croke Park, massacring 14 Irish men, women and children, and wounding many others. although the IRA's attack provoked condemnation from the British government, it was the British reprisal which solidified Irish opposition against the Crown, and which fueled world criticism of the British government.
the foregoing is only a cursory summary of the events and participants of those violent times. for greater and more colorful detail, i recommend Morgan Llywelyn's "Irish Century" series of novels -- 1916, 1921, 1949, 1972, and 1999.