Active service unit

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 members for support activities such as intelligence-gathering, acting as lookouts or moving weapons.^[5]

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 under the direct control of the IRA leadership.^[6] 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.^[7]

The exception to this reorganisation was the South Armagh Brigade which retained its traditional hierarchy and battalion structure and used relatively large numbers of volunteers in its actions.^[8] Some operations, like the attack on Cloghogue checkpoint or the South Armagh sniper squads, involved as many as 20 volunteers, most of them in supporting roles.^[9]

The smaller Republican paramilitary organisation the INLA also used the term "active service unit,^[10] as did the Loyalist paramilitary groups the Ulster Volunteer Force^[11] and Ulster Defence Association.^{[citation needed]}

• Flying Columns, terminology for some types of Irish Volunteers units of circa the 1920s
• Fireteam and Squad, terminology for functional types of modern military units of similar size