Basically, Britain ruled Ireland like a colony – like they ruled in America or in India. The Irish didn’t like this but it was much harder for them to win their independence due to Britain being right the fuck there. There were also many loyalists in Ireland, further muddying the situation, as well as Irish men in the British army fighting against the IRA.
Initially Britain decided to give them representation in parliament instead of their independence, like what N. Ireland has now. That gave rise to a conflict within the IRA, with some of the rebels wanting to end the war and accept the offer of being represented in the legislature, and some of the rebels wanting to continue to fight until Britain gave them complete independence. The separatist faction of the IRA tended to be socialists who wanted independence from England so they could make significant changes to the political and economic workings of Ireland. The others were not exactly loyalists but were capitalists who thought that home rule would be good enough to turn things around without seizing the means of production from private owners and whatnot.
Somewhere along the line, religion came into it, with England being an officially Protestant nation and Ireland being officially Catholic.
So now you had Protestants being attacked in the Republic, Catholics being attacked in the North, the IRA blowing up everything British on both sides of the line, an argument within the IRA about socialism vs. capitalism leading to them to split into to the National Army (the official standing army of the Republic of Ireland) and the irregulars and thereafter into several different factions, with the British army trying to suppress all of the above from all directions.
The West Bank situation might be pretty comparable to this in about 20 or 30 years. Currently Israel is trying to settle loyalist families in the area, displacing the Palestinians. Eventually they might reach an uneasy peace with Palestinians and Israeli living side-by-side but still hating each other. Eventually the Palestinians start to want independence but Israel is reluctant to give up the tax income that the area represents so they offer the Palestinians self-government as long as they continue to pay taxes. Some Palestinians are OK with this, but some want to evict the Israelis entirely, who they see as the cause of the Palestinians’ suffering, and seize their land and incomes and distribute them among the Palestinians as reparations… do you see where this is going?