On 21 1919 two Royal Irish Constabularly officers, escorting a load of gelignite to a quarry at soloheadbeg, Co. Tipperary, were killed in an attack led by Dan Breen and Sean Treacy. The two went under the name of the Irish Republican Army -IRA.
The IRA was, Gibney (2007) says, a "somewhat amorphous and localised organisation" that had its origins in remnants of the Irish Volunteers.
Its HQ was in Dublin and its Chief of Staff was Gaelic Leaguer Richard Mulachy. Its Director of Intelligence was another 1916 veteran Corkman Michael Collins.
Gibney (2017) adds that it was a young men's organisation. These men were often Catholics from a lower middle class background. And as volunteers they used guerilla tactics such as ambushes and assassinations.
Sinn Fein backed it up setting up what was effectively a guerilla movement. Arbitration courts were established to resolve local disputes. And by 1920 Sinn Fein had taken over most local authorities in the country.
The British Government's response was intensified repression. Martial law was imposed in eight southern counties. Plus paramilitary forces were recruited including demobilised former servicemen (Auxiliary Division) and Special Constables (Black and Tans).