The South Armagh Brigade of the Provisional Irish Republican Army (IRA) 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, roughly half of them living south of the border. It has allegedly been commanded since the 1970s by Thomas 'Slab' Murphy who is also alleged to be a member of the IRA's Army Council. Compared to other brigades, the South Armagh IRA was seen as an 'independent republic' within the republican movement, retaining a battalion organizational structure and not adopting the cell structure the rest of the IRA was forced to adopt after repeated intelligence failures.

As well as paramilitary activity, the South Armagh Brigade has also been widely accused of smuggling across the Irish border. Between 1970 and 1997 the brigade was responsible for the deaths of 165 members of British security forces (123 British soldiers and 42 Royal Ulster Constabulary (RUC) officers). A further 75 civilians were killed in the area during the conflict, as well as ten South Armagh Brigade members. The RUC recorded 1,255 bombings and 1,158 shootings around a radius of ten miles from the geographic centre of South Armagh in the same period.

South Armagh has a long Irish republican tradition. From 1919, Frank Thornton, Third Director of Operations in Michael Collins' Intelligence Staff, played a key role in organising IRA activities across South Armagh, Louth, Monaghan, Cavan, Longford and Down. Many men in the area served in the Fourth Northern Division of the Irish Republican Army during the Irish War of Independence (1919–21) and, unlike most of the rest of the Northern Ireland IRA, on the anti-Treaty side in the Irish Civil War (1922–23). Men from the area also took part in IRA campaigns in the 1940 and 1950s.

At the beginning of the Northern Ireland Troubles in August 1969, rioters, led by IRA men, attacked the RUC barracks in Crossmaglen, in retaliation for the attacks on Catholic/nationalist areas in Belfast in the Northern Ireland riots of August 1969. After the split in the IRA in that year, the South Armagh unit sided with the Provisional IRA rather than the Official IRA. The following August, two RUC constables were killed by a car bomb in Crossmaglen.

However, the IRA campaign in the area did not begin in earnest until 1971. On 7 August of that year, Harry Thornton, a 29-year-old sewage worker from South Armagh was shot and killed by the British Army while staying in a car outside Springfield Road base in Belfast, he and his coworker Arthur Murphy having been mistaken for gunmen. Murphy was lightly wounded and arrested by the RUC. The incident caused outrage among South Armagh residents, provided the IRA with many new recruits and created a hostile climate where local people were prepared to tolerate the killing of security force members.

During the early 1970s, the brigade was mostly engaged in ambushes of British Army patrols. In one such ambush in August 1972, a Ferret armoured car was destroyed by a 600 lb landmine, killing one soldier. There were also frequent gun attacks on foot patrols. Travelling overland in South Armagh eventually became so dangerous that the British Army began using helicopters to transport troops and supply its bases - a practice that had to be continued until the late 1990s. According to author Toby Harnden, the decision was taken shortly after a Saracen armoured vehicle was destroyed by a culvert bomb near Crossmaglen, on 9 October 1975. Subsequently, the British Army gave up the use of roads to the IRA in South Armagh. IRA volunteer Éamon McGuire, a former Aer Lingus senior engineer, and his team claim that they were responsible for getting the British Army "off the ground and into the air" in South Armagh. He was identified as the IRA's chief technical officer by the Central Intelligence Agency. Another noted IRA commander at that time was the commanding officer of the first battalion, Captain Michael McVerry. He was eventually killed during an attack on the RUC barracks in Keady in November 1973. Around this time IRA engineers in South Armagh pioneered the use of home-made mortars which were relatively inaccurate but highly destructive.

In 1975 and 1976, as sectarian violence increased in Northern Ireland, the South Armagh Republican Action Force, allegedly a cover-name for the South Armagh Brigade, carried out two attacks against Protestants. In September 1975 they attacked an Orange lodge in Newtownhamilton, killing five members of the lodge. Then, in January 1976, after a series of loyalist Ulster Volunteer Force (UVF) attacks on Catholic civilians in the border areas (including the Reavey and O'Dowd killings the previous day), the group shot and killed ten Protestant workmen in the "Kingsmill massacre" near Bessbrook. The workers' bus was stopped and the one Catholic worker taken aside before the others were killed. In response, the British government stated that it was dispatching the Special Air Service (SAS) to South Armagh, although the SAS had been present in the area for many years. While loyalist attacks on Catholics temporarily declined afterwards and many Protestants became more reluctant to help the UVF, the massacre caused considerable controversy in the republican movement.

By the end of the 1970s, the IRA in most of Northern Ireland had been restructured into a cell system. South Armagh, however, where the close rural community and family connections of IRA men diminished the risk of infiltration, retained its larger "battalion" structure. On 17 February 1978 the commander of the 2nd Battalion Royal Green Jackets, Lieutenant Colonel Ian Corden-Lloyd, was killed and two other soldiers injured when the Gazelle helicopter he was travelling in was attacked by an IRA unit near Jonesborough. At that moment, a gun battle was taking place on the ground between British soldiers and members of the South Armagh Brigade. The helicopter crashed while taking evasive manoeuvres after being fired at from the east side of Edenappa road. Corden-Lloyd's subordinates had been accused of brutality against Catholic civilians in Belfast in 1971. In August 1979, a South Armagh unit killed 18 soldiers in the Warrenpoint ambush. This was the biggest single loss of life inflicted on the British Army in its deployment in Northern Ireland (Operation Banner).