Active Service Unit
An active service unit was a Provisional Irish Republican Army
Provisional Irish Republican Army
The Provisional Irish Republican Army is an Irish republican paramilitary organisation whose aim was to remove Northern Ireland from the United Kingdom and bring about a socialist republic within a united Ireland by force of arms and political persuasion...

 (IRA) cell
Clandestine cell system
A clandestine cell structure is a method for organizing a group of people in such a way that it can more effectively resist penetration by an opposing organization. Depending on the group's philosophy, its operational area, the communications technologies available, and the nature of the mission,...

 of five to eight members, tasked with carrying out armed attacks. In 2002 the IRA had about 1,000 active members of which about 300 were in active service units.
In 1977, the IRA moved away from the larger conventional military organisational principle owing to its perceived security vulnerability. In place of the battalion structures, a system of two parallel types of unit within an IRA Brigade was introduced. Firstly, the old "company" structures were used to supply auxiliary member for support activities such as intelligence gathering, acting as lookouts or moving weapons.

The bulk of attacks from 1977 onwards were the responsibility of a second type of unit, the ASU. To improve security and operational capacity these ASUs were smaller, tight-knit cells, usually consisting of five to eight members, for carrying out armed attacks. The ASU's weapons were controlled by a quartermaster
Quartermaster refers to two different military occupations depending on if the assigned unit is land based or naval.In land armies, especially US units, it is a term referring to either an individual soldier or a unit who specializes in distributing supplies and provisions to troops. The senior...

 under the direct control of the IRA leadership. By the late 1980s and early 1990s, it was estimated that the IRA had roughly 300 members in ASUs and approximately 450 serving in supporting roles.

The exception to this reorganisation was the South Armagh Brigade
Provisional IRA South Armagh Brigade
The South Armagh Brigade of the Provisional Irish Republican Army operated during the Troubles in south County Armagh. It was organised into two battalions, one around Jonesborough and another around Crossmaglen. By the 1990s, the South Armagh Brigade was thought to consist of about 40 members,...

 which retained its traditional hierarchy and battalion structure and used relatively large numbers of volunteers in its actions. Some operations, like the attack on Cloghogue checkpoint
Attack on Cloghogue checkpoint
The attack on Cloghogue checkpoint was an unconventional bomb attack carried out on 1 May 1992 by the Provisional Irish Republican Army against a British Army permanent vehicle checkpoint...

 or the South Armagh sniper squads
South Armagh Sniper (1990-1997)
The South Armagh Sniper is the generic name given to the members of the Provisional Irish Republican Army's South Armagh Brigade who conducted a sniping campaign against British security forces from 1990 to 1997....

, involved as many as 20 volunteers, most of them in supporting roles.
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