system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why did the Troubles happen in Northern Ireland, and why was it at times so violent?
gpt
To understand the modern troubles, and to appreciate why it was so violent you have to look at the history as far back as the 12th century when Ireland was invaded by the Normans. This was the start of 'direct English rule' this established a deep running hatred in many native of the native Irish people, who felt they were being oppressed and they hadn't the cohesion at that time to fight back

Matters didnt get any better and when Henry VIII came to the throne in England and started the English reformation, and his own branch of Protestantism, he proclaimed himself King of Ireland and tried to impose the Protestant beliefs on Irish people - The majority of whom were catholic peasants.
The Reformation failed in Ireland and this lead to a campaign known as the 'Plantation' in which many English and Scottish Protestants were promised some of the most fertile and prosperous land in Ireland, at the expense of many of the native Irish farmers. At this point tensions had increased between the Protestant and Catholic sides ( Who were typically seen as English and Native Irish respectively - and sometimes still are by the more narrow minded people) the tension culminated in various re- occurring conflicts that became part of the tapestry of Irish, and then Northern Irish culture.


Again things did not get any better, Irish people were considered second class to Protestants, and were not even granted full rights until the Catholic Emancipation in 1829.
The Irish potato famine had further negative effect on relations also as the English government of the time had an economic policy of "Laisser Faire" - meaning to 'let be'. Implying they would not get involved in economic issue that they believed should right themselves in time. 

Tensions and conflicts came to a head In 1922, after the Irish War of Independence and the Anglo-Irish Treaty, most of Ireland seceded from the United Kingdom to become the independent Irish Free State, which after the 1937 constitution, began to call itself Ireland. 
The six northeastern counties, known as Northern Ireland, remained within the United Kingdom. The Irish Civil War followed soon after the War of Independence. The history of Northern Ireland has since been dominated by sporadic sectarian conflict between (mainly Catholic) Irish nationalists and (mainly Protestant) unionists. This conflict erupted into the Troubles in the late 1960s.

So to understand why it may have been so violent at times, you have to appreciate that this conflict had been building up and even ongoing for some 700 years and while many counties of Ireland split from the UK, many 'Catholic Irish' people were stuck in a UK controlled Northern Ireland and were reluctant to leave their homes, and this sentiment was shared by the 'Protestant British' who at this point had been living in Ireland for many years also.