Though arguably by midway through it had become a cycle of factional violence, it originated in a civil rights movement parallel to that of the US in the 1960s. The following should resonate with Americans:

• Catholics were discriminated against in employment, including government employment
• the police forces, even in Catholic neighborhoods, were almost entirely Protestant, and seen as tolerating Protestant harassment of Catholics
• gerrymandering of voting districts led to Protestant over-representation even in heavily Catholic areas
• a biggie, all the rest of the UK had gone over to each adult getting one vote, but NI it was still one vote to each household. Protestants tended to live in nuclear families, so mom and dad got one vote total. But for Catholics who tended to live in extended families, mom, dad, unemployed uncle, grandpa, and two adult sons all got one vote total for the whole house.

It spiraled into violence basically like this: Catholics go protest, things get heated and they fight Protestants, cops wade in and whomp almost entirely on Catholics (or at least are so perceived). Some Catholics get more militant and arm up, and the largely-defunct IRA rises again. Protestants freak out and start forming their militias and violence gets more deliberate. UK sends in the Army to keep the peace, all rejoice, but then the Army comes to be seen as biased to the Protestants, so now the Catholics feel they're under military occupation to keep them down, so violence just keeps spiraling.

That's my over-simplified take, saving the thread because it's a fascinating topic.

to put it simply the main cause was the divide of people into the groups of loyalists who where protestants and wanted northern Ireland to remain part of the UK and the republicans who where Catholics and wanted a independent Ireland.

The conflict really started during a campaign by the republicans to end discrimination against Catholics minority by the loyalist protestant government and police force. the police used violence to often stop any republican uprisings ans where accused of police brutality and it was also met with violence from loyalists who believed it was a republican front.

Nothing much really happened until 1966 which saw the creation of the Ulster volunteer group a paramilitary group and the first to be formed in the troubles. but the first major event was not until 1968 and the battle of Bogside which was a riot organised by the residents of the Bogside area against the Ulster constabulary.

This was what happened at the very start of the troubles and the battle of Bogside is what really kicked of violence across northern Ireland.