Casualties
This is a summary. For a detailed breakdown of casualties caused by and inflicted on the IRA see Provisional IRA campaign 1969-1997#Casualties
The IRA was responsible for more deaths than any other group during the Troubles. Two very detailed studies of deaths in the Troubles, the CAIN project at the University of Ulster, and Lost Lives, differ slightly on the numbers killed by the IRA, but a rough synthesis gives a figure of 1,800 deaths. Of these, roughly 1,100 were members of the security forces: British Army, Royal Ulster Constabulary and Ulster Defence Regiment; between 600 and 650 were civilians and the remainder were either loyalist or republican paramilitaries (including over 100 IRA members accidentally killed by their own bombs or shot after being exposed as security force agents).
The IRA lost a little under 300 members killed in the Troubles. In addition, roughly 50–60 members of Sinn Féin were killed. Far more common than the killing of IRA volunteers, however, was their imprisonment. Journalists Eamonn Mallie and Patrick Bishop estimate in their book The Provisional IRA that roughly 8,000 people passed through the ranks of the IRA in the first 20 years of its existence, many of them leaving after arrest (senior officers are required to surrender their post after being arrested), retiring from the armed campaign or "disillusionment". They give 10,000 as the total number of past and present IRA members at that time.
Read more about this topic: Provisional Irish Republican Army