A synopsis of this topic is pretty difficult to do, since you're talking about centuries of ongoing political and religious tribalism in Northern IReland between Irish Republicans and Nationalists who want to end partition and unite with the Republic of Ireland, and Unionists and Loyalists who want to maintain the current arrangement of being a part of the United Kingdom. The Loyalists and Unionists are predominately Protestant and the Republicans and Nationalists are overwhelmingly Roman Catholic, but it is not a religious dispute; they are not arguing about doctrine but constitutional identity.

The Partition of Ireland in the 1922 created a majority- Roman Catholic self-governing Ireland in the South, with its own Parliament, and a majority-Protestant, self governing Northern Ireland, also with it's own Parliament. The southern Irish Parliament gradually ended the constitutional arrangement with the United Kingdom, becoming a Republic in 1949. The northern Parliament (referred to as Stormont, based on the Belfast suburb it was based), exercised its right to remain a part of the UK.

However, the Stormont parliament was a kind of absolute majoritarian framework, which meant that Unionists (functioning through the Official Unionist Party) were able to gradually turn the Northern Parliament into a conservative, aristocratic one-party state via gerrymandering and plural voting. Gerrymandering reduced even the proportional electorate of local councils to near-universal majority Protestant councils, while plural voting meant that only ratepayers and spouses held the franchise, and owners of businesses (largely Unionist) were allowed a second vote.

Control of the councils also meant control of housing, and from 1921 to the end of the Northern Ireland Parliament, discrimination in housing against Roman Catholics was rampant; remember that this also had an influence in voting, since only the ratepayer and spouse held the franchise for the address. In Catholic areas, there may be four or five adults living in a house and only one or two of them actually held a vote. Meanwhile, discrimination in the police department created an "us vs. them" mentality.

In and amidsts all of this lurked the Irish Republican Army, who, despite its numbers being quite small and taking a sharp Marxist bent in the 1960s, believed itself to be the only legitimate government for the entire island, and engaged in a series of terrorist bombings and shootings, and became a thorn in the side to both the Dublin government and the Stormont government.

IN 1968, the Northern Ireland Civil Rights Association held its first march protesting discrimination in housing. Stormont saw this as a Nationalist/Catholic attempt to undermind the Protestant/Unionist government, and any leeway on Catholic civil rights would ultimately elad to Union with Dublin. The first march went off quietly, but following marches in Derry lead to conflict with the Unionist-dominated Police after marches and counter-marches were banned and protestors marched anyway. MPs were assaulted by the police, baton charges, etc., and is largely considered the start of the Troubles. Despite some concessions to NICRA by Stormont (one man, one vote, for example), the tension continued and culiminated with the Battle of the Bogside in Derry in 1969, deployment of the British Army into the province in August, and the introduction of internment without trial in 1971.

The IRA split between the avowedly Marxist Official IRA and the more militant, northern-based Provisional IRA in 1970. By this time, power in the UUP had shifted to more militant leaders such as Brian Faulkner and Ian Paisley, creating even more tension in Northern Ireland. In 1971, barricades and no-go areas existed in Nationalist areas of Derry and Belfast, and patrol and civil order in those areas passed to the Provisional IRA, though direct conflict between PIRA, the Army and the Police was limited. Political violence continued to deteriorate until January 1972, when on Bloody Sunday, British paratroopers shot and killed 14 peaceful NICRA marchers and wounded 14 more in Derry. Three months later, the Stormont Government was eliminated and direct rule under a SEcretary of State for Northern Ireland from Westminster was introduced. For most historians, Bloody Sunday signals the end of the peaceful protest movement for Civil Rights in Northern Ireland, and turns towards a military conflict between the following groups:

1. Republicans who believed in violent conflict with the British Army to unite Ireland. Sinn Fein, Provisional IRA, the Irish Republican Liberation Army, are all Republican groups
2. Nationalists who believed in achieving civil rights through the ballot box, a negotiated settlement to the Army's deployment, and eventual union with Ireland through the democratic process. The Social Democratic Labour Party
3. Unionists who believe in maintaining the current constitutional arrangement, though they are split on how this best achieved, but largely through constitutional processes, even if constitutionalism means a vulgar majoritarian understanding. The Ulster Unionist Party is in this group, and in portions of the Ulster Democratic Party (now the largest unionist party in Northern Ireland)
4. Loyalists, who believe in violent conflict with Republicans as well as Unionists and constitutional bodies that move towards any untying of the relationship between the UK and northern Ireland. The Ulster Defense Force, the Ulster Volunteer Force, Red Hand Brigade, etc. usually fall in this category.

I'll skip the time period between 1972 to 1998, since it tends to get bogged down in tit-for-tat bombings, failed negotiations and on-and-off ceasefires. IN 1998, though, a breakthrough between the UUP leadership and SDLP leadership created the movement towards a lasting peaceful, constitutional solution to the Troubles. Essentially, a Northern Ireland Assembly would be created and the executive would power-shared throughout the parties that had accepted the principles of the Belfast Agreement. Both the Loyalist and the Republican paramilitary organs agreed to a ceasefire, end to hostilities and the decommissioning of weapons, and the police were to be reformed. Sinn Fein dropped its traditional absentionist policy and took seats in a UK institution for the first time. There were hiccups, but largely the agreement reached 20 years ago is still in place.

Short enough?

Citations: Tim Pat Coogan (2002). The Troubles: Ireland's Ordeal and the Search for Peace. Palgrave Macmillan. Tim Pat Coogan (2002). The IRA. Palgrave Macmillan. Brendan O'Brien (1999). The Long War: The IRA and Sinn Féin Syracuse, New York: Syracuse University Press. Conflict Archive on the Internet, University of Ulster