The Provisional Irish Republican Army (Provisional IRA), officially known as the Irish Republican Army (IRA; Irish: Óglaigh na hÉireann) and informally known as the Provos, was an Irish republican paramilitary force that sought to end British rule in Northern Ireland, facilitate Irish reunification and bring about an independent republic encompassing all of Ireland. It was the most active republican paramilitary group during the Troubles. It argued that the all-island Irish Republic continued to exist, and it saw itself as that state's army, the sole legitimate successor to the original IRA from the Irish War of Independence. It was designated a terrorist organisation in the United Kingdom and an unlawful organisation in the Republic of Ireland, both of whose authority it rejected.

The Provisional IRA emerged in December 1969, due to a split within the previous incarnation of the IRA and the broader Irish republican movement. It was initially the minority faction in the split compared to the Official IRA but became the dominant faction by 1972. The Troubles had begun shortly before when a largely Catholic, nonviolent civil rights campaign was met with violence from both Ulster loyalists and the Royal Ulster Constabulary (RUC), culminating in the August 1969 riots and deployment of British soldiers. The IRA initially focused on defence of Catholic areas, but it began an offensive campaign in 1970 that was aided by external sources, including Irish diaspora communities within the Anglosphere, and the Palestine Liberation Organization and Libyan leader Muammar Gaddafi. It used guerrilla tactics against the British Army and RUC in both rural and urban areas, and carried out a bombing campaign in Northern Ireland and England against military, political and economic targets, and British military targets in mainland Europe. They also targeted civilian contractors to the British security forces. The IRA's armed campaign, primarily in Northern Ireland but also in England and mainland Europe, killed over 1,700 people, including roughly 1,000 members of the British security forces and 500–644 civilians.

The Provisional IRA declared a final ceasefire in July 1997, after which its political wing Sinn Féin was admitted into multi-party peace talks on the future of Northern Ireland. These resulted in the 1998 Good Friday Agreement, and in 2005 the IRA formally ended its armed campaign and decommissioned its weapons under the supervision of the Independent International Commission on Decommissioning. Several splinter groups have been formed as a result of splits within the IRA, including the Continuity IRA, which is still active in the dissident Irish republican campaign, and the Real IRA.

The original IRA was formed in 1913 as the Irish Volunteers, at a time when all of Ireland was part of the United Kingdom. The Volunteers took part in the Easter Rising against British rule in 1916, and the War of Independence that followed the Declaration of Independence by the revolutionary parliament Dáil Éireann in 1919, during which they came to be known as the IRA. Ireland was partitioned into Southern Ireland and Northern Ireland by the Government of Ireland Act 1920, and following the implementation of the Anglo-Irish Treaty in 1922 Southern Ireland, renamed the Irish Free State, became a self-governing dominion while Northern Ireland chose to remain under home rule as part of the United Kingdom. The Treaty caused a split in the IRA, the pro-Treaty IRA were absorbed into the National Army, which defeated the anti-Treaty IRA in the Civil War. Subsequently, while denying the legitimacy of the Free State, the surviving elements of the anti-Treaty IRA focused on overthrowing the Northern Ireland state and the achievement of a united Ireland, carrying out a bombing campaign in England in 1939 and 1940, a campaign in Northern Ireland in the 1940s, and the Border campaign of 1956–1962. Following the failure of the Border campaign, internal debate took place regarding the future of the IRA. Chief-of-staff Cathal Goulding wanted the IRA to adopt a socialist agenda and become involved in politics, while traditional republicans such as Seán Mac Stíofáin wanted to increase recruitment and rebuild the IRA.

Following partition, Northern Ireland became a de facto one-party state governed by the Ulster Unionist Party in the Parliament of Northern Ireland, in which Catholics were viewed as second-class citizens. Protestants were given preference in jobs and housing, and local government constituencies were gerrymandered in places such as Derry. Policing was carried out by the armed Royal Ulster Constabulary (RUC) and the B-Specials, both of which were almost exclusively Protestant. In the mid-1960s tension between the Catholic and Protestant communities was increasing. In 1966 Ireland celebrated the 50th anniversary of the Easter Rising, prompting fears of a renewed IRA campaign. Feeling under threat, Protestants formed the Ulster Volunteer Force (UVF), a paramilitary group which killed three people in May 1966, two of them Catholic men. In January 1967 the Northern Ireland Civil Rights Association (NICRA) was formed by a diverse group of people, including IRA members and liberal unionists. Civil rights marches by NICRA and a similar organisation, People's Democracy, protesting against discrimination were met by counter-protests and violent clashes with loyalists, including the Ulster Protestant Volunteers, a paramilitary group led by Ian Paisley.

Marches marking the Ulster Protestant celebration The Twelfth in July 1969 led to riots and violent clashes in Belfast, Derry and elsewhere. The following month a three-day riot began in the Catholic Bogside area of Derry, following a march by the Protestant Apprentice Boys of Derry. The Battle of the Bogside caused Catholics in Belfast to riot in solidarity with the Bogsiders and to try to prevent RUC reinforcements being sent to Derry, sparking retaliation by Protestant mobs. The subsequent arson attacks, damage to property and intimidation forced 1,505 Catholic families and 315 Protestant families to leave their homes in Belfast in the Northern Ireland riots of August 1969 The riots resulted in 275 buildings being destroyed or requiring major repairs, 83.5% of them occupied by Catholics. A number of people were killed on both sides, some by the police, and the British Army were deployed to Northern Ireland. The IRA had been poorly armed and failed to properly defend Catholic areas from Protestant attacks, which had been considered one of its roles since the 1920s. Veteran republicans were critical of Goulding and the IRA's Dublin leadership which, for political reasons, had refused to prepare for aggressive action in advance of the violence. On 24 August a group including Joe Cahill, Seamus Twomey, Dáithí Ó Conaill, Billy McKee, and Jimmy Steele came together in Belfast and decided to remove the pro-Goulding Belfast leadership of Billy McMillen and Jim Sullivan and return to traditional militant republicanism. On 22 September Twomey, McKee, and Steele were among sixteen armed IRA men who confronted the Belfast leadership over the failure to adequately defend Catholic areas. A compromise was agreed where McMillen stayed in command, but he was not to have any communication with the IRA's Dublin based leadership.

The IRA split into "Provisional" and "Official" factions in December 1969, after an IRA convention was held in Boyle, County Roscommon, Republic of Ireland. The two main issues at the convention were a resolution to enter into a "National Liberation Front" with radical left-wing groups, and a resolution to end abstentionism, which would allow participation in the British, Irish, and Northern Ireland parliaments. Traditional republicans refused to vote on the "National Liberation Front", and it was passed by twenty-nine votes to seven. The traditionalists argued strongly against the ending of abstentionism, and the official minutes report the resolution passed by twenty-seven votes to twelve.

Following the convention the traditionalists canvassed support throughout Ireland, with IRA director of intelligence Mac Stíofáin meeting the disaffected members of the IRA in Belfast. Shortly after, the traditionalists held a convention which elected a "Provisional" Army Council, composed of Mac Stíofáin, Ruairí Ó Brádaigh, Paddy Mulcahy, Sean Tracey, Leo Martin, Ó Conaill, and Cahill. The term provisional was chosen to mirror the 1916 Provisional Government of the Irish Republic, and also to designate it as temporary pending ratification by a further IRA convention. Nine out of thirteen IRA units in Belfast sided with the "Provisional" Army Council in December 1969, roughly 120 activists and 500 supporters. The Provisional IRA issued their first public statement on 28 December 1969, stating: