Why did the Troubles actually start? Was the UK actually an oppressive brutal state as late as 1980s/90s?

The issues for Catholics in Northern Ireland go back some hundreds of years but the modern day "Troubles" has roots in the establishment of the Northern Ireland border by the Government of Ireland Act 1920 passed in Westminster. This divided Ireland into two states with Northern Ireland remaining within the UK.

Even prior to the act being passed Catholics were expelled from one of the major employers in the city of Belfast, Harland and Wolffe. This was mainly the work of the Belfast Protestant Association who called on Protestant and Unionist workers to drive out Catholic workers. This was soon followed by expulsions from other major employers in the city, Musgraves, Mackies Foundry and several linen mills. Riots followed and approximately 18 people were killed. Hundreds of mainly Catholic families were driven from their homes. Little was done to help the situation and Northern Ireland settled into a situation where Catholics found themselves to be second class citizens. Employment, housing and voting rights were denied to them. Careful gerrymandering made the Catholic vote impotent even in areas - Fermanagh and Tyrone - which were majority Catholic.

In the 1960s influenced by the civil rights movement in the United States a civil rights group - the Northern Ireland Civil Rights Association - was formed to highlight these issues. In Aug 1968 the first civil rights march was organised. This was immediately met by violence from the Protestant paramilitary group, the Ulster Volunteer Force, who attacked marchers. This paramilitary group had roots in 1913 and reorganised in 1966 to ‘protect’ against any concession for Catholics who were beginning to become vocal in their demands for economic and political participation. The Orange Order also organised against the marches. This violent intimidation continued for other civil rights marches. The initial response of the British Government and the Belfast government was to ignore the demands of the marchers or even to agree to discuss the issues. Instead the Belfast government decided to ban all civil rights marches. This happened in the face of Orange marches, which had a long history of provocation against Catholics by marching through Catholic areas, being allowed to proceed.

A critical point was reached in when the Derry Civil rights branch decided to go ahead with a march that had been banned. Initially the march was peaceful but the police, the Royal Ulster Constabulary [RUC], moved in on the march to break it up using batons against the marchers. The result was a disaster. Two days of riots followed. This incident is regarded as the start of the modern day 'troubles' in Northern Ireland.

Later that summer a Protestant march was permitted to go ahead in spite of warning from many including business owners that it would lead to disaster. Some 15,000 Unionists turned out to march along the walls of Derry and went close to the Catholic Bogside area. The situation got out of control as Catholics erected barriers along their street to prevent the marchers and the police from entering Catholic homes. The RUC then used an armoured car to break down the Catholic erected barriers and hurled gas grenades. More than 3,000 Catholics were driven from their homes by the police. Word was sent from Derry to the Civil Rights association in Belfast about what was happening in Derry and soon Belfast and other cities and towns in Northern Ireland were in riot. This news was blazed on television screens throughout Ireland and the government in Dublin demanded of the British government that something be done to quell the violence against Catholics. The Irish Taoiseach Jack Lynch went on Irish television to announce that Irish troops were being sent along the border to assist Catholics who needed help should they wish to cross the border. He also called for UN intervention. The British government was angered at Lynch’s statement and his request for UN intervention was rejected. Instead they sent in British troops into NI.

Notably absent from all this is the IRA. That is because they were a depleted body by this time. However, with the violence against Catholics and the intervention of British troops, they emerged from the shadows. In Aug of 1969 the small body of IRA in Belfast – estimated to be less than 50 members – made contact with HQ in Dublin asking for assistance in obtaining arms. At their Ard Fheis [general meeting] in Dublin n Dec 1969 the IRA split on how to proceed. The Official IRA wanted no violent intervention. The Provisional IRA was formed and chose to proceed to obtain arms.

The rest that followed was over 30 years of violence in NI as the ‘Provos’ clashed with the Protestant paramilitaries the UVF, the British Army and NI police.

Edit: typos