For the purposes of what we need to know, the history of Ireland starts in 1167 with the Norman invasion. From this point to the English Civil War, the Normans/English forces had control over variable parts of Ireland, usually focusing on the eastern coast in and around Dublin. The English would bring English troops, English lords, and English laws to rule over the Irish.
In the 1640's, During the English Civil War, the Protestant government of Cromwell invaded Ireland to gain control over the whole of the Island, and complete the overthrow of James, the King who found support on the Island. A series of brutal reforms took place where Irish and Catholic landlords were replaced with Protestant landlords, many from England. Finally, in 1690, the Battle of the Boyne took place. It's celebrated every year on July 12th. This battle cemented Protestant and English rule in Ireland.
This began a period where the 5% of the population who were members of the Church of Ireland (Anglicans) ruled over the Island. This began the famous period of famines, and due in large part due to absentee landlords, was very difficult on the Irish Catholic population.
In the 1870's, the Irish Parliamentary Party was formed. This party would be the first, perhaps in the world, to use the modern whip system. The party would agree on a position with all it's members, and then all members, even those who were privately against the position, would vote for it in Parliament. This block of votes ensured the party had the power to sway the government at times of minority rule.
Finally, in 1898, a new local government bill was passed that allowed for the mostly catholic residents to have a true say in how they were governed.
Debate over home rule (IE Sovereignty) lead to conflict between nationalists and unionists. By 1912, the government in London agreed to give Ireland home rule. The granting of home rule was delayed during WW1.
In 1916, fed up with waiting, nationalists in Dublin held what would be known as the easter rising, an armed revolt, which was put down harshly by the British. 16 leaders of the revolt were executed, and this shifted mainstream public opinion in Ireland towards Independence.
The December 1918 elections saw Sinn Fein, the pro-Independence party, win 73 of the 105 seats. SF declared that a meeting of all Irish MPs would take place in Dublin, and at the meeting, declared Ireland independent, and that the 105 MPs elected to Westminster, would in fact, sit as the first Dail (Irish Parliament).
22 Unionists, 3 Labour Unionists, and 1 Independent Unionist refused to sit the Dail or even recognize it. The 6 members of the Irish Parliamentary Party that were elected also refused to participate.
You may also notice a geographic trend. In what would later become Northern Ireland, Irish Unionists won 20 seats, joined by 3 Labour Unionists; while the party would only win 2 in the south, who They would be joined by an Independent Unionist. Sinn Fein would win 71 seats in the south, but only 2 in the North. The IPP meanwhile won 4 in the north but 2 in the south. This puts 2 Northern MPs on the "wrong" side of the border, and 5 southern MPs on the "wrong" side.
In the south, Arthur Samuels, Irish Unionist, would go on to serve as a judge in Northern Ireland. Independent Unionist, Robert Woods, retired from Politics; both these men from the University seat in Dublin (the circles on the map) Irish Unionist Maurice Dockrell would retire from politics, but his son, Henry Morgan Dockrell, would serve as a TD, and his sons, would also serve. Edward Kelly, IPP member from the province of Ulster (but outside Northern Ireland) would also retire once his term expired. William Redmond was the IPP member from Waterford. He had served in WW1, and his father, John, had been IPP leader. Redmond would serve his term as MP until 1922, when the term ended, and then seek election to the Dail, where he won as an Independent Nationalist.
In the north, both SF members had simultaneously been elected in other seats, and as such, continued their political life there. Others had also done this in the party; This means that while 73 seats were won by SF, only 69 members were elected, all of which had seats in what would become the Republic of Ireland.
In 1920, an act was passed by the British to split Ireland in two. The north would be retained by the UK while the south would become an independent dominion. In 1921, elections were held in both parts of Ireland. In the south, SF won 124 seats, while Independent Unionists won 4. While these 4 members would refuse to sit in the 2nd Dail, they would all become active in the south, with half being re-elected to (and sitting in) the 3rd Dail, and the other two becoming a Provost of the University, and a Judge respectively. In the north, the Ulster Unionist Party (successor to the Irish Unionist Party) won 40 seats, while SF won 6, and the Nationalists (successor to the IPP) won 6. 5 of the 6 elected members for SF were already TDs in the south, and the 6th did not choose to 'test' if he would be allowed into the Dail.
This period would see the south fall into civil war, fighting over if Ireland should accept not only partition, but the idea of dominion status (meaning the King remains) but the south is not our focus, the north is.
Ireland has 4 provinces. Unlike Canada, these "Provinces" have no power, and are geographic areas only. Northern Ireland is made up of 6 of the 9 counties of Ulster. As such, many unionists consider Northern Ireland and Ulster to be one in the same, while nationalists consider such use of the term offensive, as only 2/3rds of Ulster are in Northern Ireland.
The Ulster Unionist Party we know today is a direct successor to the Irish Unionist Party that contested these elections. At the time, the party was seen as a local branch of the Conservative Party, and as such, the UUP has some ties to the Conservatives.
The civil war in the south, and the loss of interest of SF in the North would lead to the disappearance of the party. Sinn Fein would not re-appear until 1954.
Starting in the mid 1960's, a civil rights movement started in Northern Ireland. The movement had evidence that Catholics were less likely to be hired, and that social housing was allocated to Protestants ahead of Catholics on the waiting lists. It demanded reform of the RUC, the police force, saying it was 90% Protestant, and demanded repeal of the Special Powers Act, which allowed the police to arrest without warrant, and was used against Catholic Nationalists. It also demanded an end to Gerrymandering, and the introduction of one man one vote to local elections.
I've drawn this map to help explain. Areas in dark Green were those with Catholic majorities, where the local council also had a Catholic majority; but areas in light green, despite a (usually significant) majority of residents being Catholic, were controlled by Protestants. Laws and rules about who could vote (for example, limiting the vote only to homeowners) were used to keep Catholics out of office.
In response, many in the Protestant community said that this was a front for republicans to unite all of Ireland. In 1966, parades were held to mark the 50th anniversary of the Easter Rising. In response, Ian Paisley set up the Ulster Constitution Defence Committee, which set up a troublemaking wing called the Ulster Protestant Volunteers, or UPV. At the same time, in Belfast, the terrorist Ulster Volunteer Force, or UVF, was set up (they would later found the Progressive Unionist Party)
This entire period had seen low-level terrorist activity from the IRA with the objective of uniting all of Ireland. Both the UPV and UVF declared the IRA as their main enemy, but both also opposed, and were opposed by, Terence O'Neill, the protestant Prime Minister of Northern Ireland, and leader of the UUP. The UVF began a concentrated terrorist campaign of bombing houses of catholics, saying that it was opposing anyone who allied with the IRA. UPV members also started physically attacking Catholics who marched in protest of their conditions, while the RUC did little to nothing to intervene.
O'Neill tried to deal with the situation, but his concessions were rejected as too little by the nationalists and too much by unionists. Protests would continue and riots would break out from time to time, including a 2 day period of fighting between Loyalists, backed by the police, and Catholics in Londonderry. This caused the IRA to become more involved in Derry (the Irish name of the city) and other areas.
During this period, the IRA split in two, both calling themselves the IRA. The Provisional Irish Republican Army, or PIRA (or Provos for short) began to arm and prepare for conflict.
On January 30th, 1972, a group of mostly Catholic protesters gathered in Derry for a march scheduled at 2:45pm. Martin McGuinness was in attendance. Youths began throwing stones at the gathered police and British Army members, who responded with rubber bullets and water cannons. This was not unusual and was a common sight. At 3:55pm, some in the crowd spotted paratroopers on the 3rd floor of an abandoned building. Protesters began throwing rocks at the windows. The soldiers responded with bullets.
Over the next 30 minutes 26 civilians were shot by the paratroopers. 13 died on the scene, and 1 died later in hospital. This would mark what became known as "The Troubles". This, was Bloody Sunday.