Over a period of 40 years, there were hundreds of murders and bombings in the North of Ireland instigated by Republican (pro Irish) and Unionist (pro UK) supporters. The victims were often murdered brutally as suspected informants of the British spy agencies. Weapons and money were funneled to the Republicans from ties to the USA, where almost 90% of the population claim Irish roots.
This mixture of violence, informants/agents, money, low prospects and British soldiers patrolling the streets was a period of time referred to as The Troubles. The Troubles ended for the most part in 1998 with something called The Good Friday Agreement which gave Northern Ireland a special self-governed status and removed borders between the North and South. Two referendums were passed with majorities of 71% and 94% in support of the agreement.
Despite the end of armed violence in Northern Ireland, the Good Friday Agreement can only be seen as a failure because it does not allow The Conservative Party to make a Brexit deal that severs Ireland in two again with a hard border to lock out the Euroweenies in the South. A majority in Northern Ireland rejected any form of Brexit but English voters carried it by 52-48 so it must be respected at all costs.
A return to The Troubles seems very likely.
Which means it's popcorn time! Pick a side and gather round the TV for 40 years. One the one hand we have the British with their legendary spy apparatus and an experienced army and on the other we have a rag-tag Irish militia without government backing (except unofficially the Americans) but no scruples about bombing civilians. Who's going to be the winner?