An active service unit (ASU; Irish: aonad seirbhíse cogúla)[1][2] was a Provisional Irish Republican Army (IRA) cell of four to ten members, tasked with carrying out armed attacks.[3] In 2002, the IRA had about 1,000 active members of which about 300 were in active service units.[4] The concept was first pioneered by Tom McEllistrim and other members of the Irish Republican Army.