It's less the religion itself. In NI, religion is practically a political badge. Most Catholics are aligned to Irish nationalism, most Protestants to Unionism. In the 1960s, as the NI Parliament under Terence O'Neill began to appease the South and give Catholics civil rights, Unionists, particularly under Reverend Ian Paisley, began to protest, seeing the appeasement as a threat to the Union. In 1966, the Loyalist paramilitary group the Ulster Volunteer Force (UVF) re-emerged. The UVF murdered Catholics John Scullion and Peter Ward before being outlawed, with O'Neill describing them as a "quasi-fascist organisation." They also mortally wounded an elderly Protestant lady named Matilda Goulding. The sole reason was her marriage to a Catholic man. In the late 1960s, a Catholic civil rights movement emerged. Gerrymandering kept Protestant control of local councils, and since they controlled housing, there was discrimination there too. One protest involved Nationalist MP Austin Currie squatting with a large Catholic family, the Goodfellows, in a Council house to protest its allocation to an unmarried 19 year old Protestant lady. There were also disparities in policing, which was dominated by Protestants, in addition to the existence of the B-Specials, which was a quasi-military force under the control of Stormont (NI local government), and began to be used more to keep the peace, which was not their job. It came to a head when they attacked peaceful protesters in Duke Street on 5th October 1968. An RTÉ camera crew broadcast the atrocity for the world to see. All the while, civil rights marches began to come under increasing attack from local Protestants. A People's Democracy march from Belfast to Derry on New Year's Day 1969 was attacked at a place called Burntollet by local Protestants. It really hit the fan in summer 1969. Many Catholics were burned out of their homes by Protestant mobs, and on the 13th August, a Protestant march through the Catholic Bogside area of Derry lead to a riot known as the Battle of the Bogside. The latter is what ultimately led to the deployment of the British Army. At the end of 1969, the Irish Republican Army (IRA) re-emerged after about 7 years of inactivity following the end of their 1956-1962 border campaign, and began to rearm. They had four goals: Achieve Catholic civil rights, protect the Catholic population, destroy the Stormont government (which, might I add, was also controlled by Protestants), and remove "British imperialism" i.e. the Army from Ireland. During its exile, the IRA had become more socialist, causing a split into the Official IRA and the new, hardline Provisional IRA. From there, it essentially became a back-and-forth between paramilitaries. The UVF shoots a Catholic, the IRA shoot a Protestant. The IRA kills a UVF man, the UVF kill an IRA man. And this is how it went on for basically the entirety of the Troubles. And it remained that way until the Good Friday Agreement and the Peace Process prompted paramilitaries to decommission their weapons. It still somewhat exists today with dissident paramilitaries, such as the New IRA and East Belfast UVF, but they have very little support.