When the UK left the EU with Brexit there were hard choices to be made in Northern Ireland. The Good Friday Agreement which has kept sectarian violence nonexistent was contingent on there being a frictionless border between Northern Ireland and Ireland. As members of the EU have the right to travel without customs, visas, etc. it was a logical that there would be a frictionless border so Irish and Northern Irish citizens could move without restriction between the two countries.
With Brexit, the only way to maintain the frictionless border between Northern Ireland and Ireland was to treat Northern Ireland as still part of the EU. This meant creating a border in the Irish Sea between Northern Ireland and the rest of the UK.
The Loyalist (i.e. Protestant or pro-UK) paramilitaries are being encouraged by the Democratic Unionist Party (DUP), which has always hated the Good Friday Agreement, to back out and at least potentially use violence to keep Northern Ireland part of the UK (another part of the Good Friday agreement is an automatic referendum if polling indicates there is greater than 50% of the Northern Irish who wish to reunite with Ireland).
If the Loyalist paramilitaries withdraw, it is inevitable that the Nationalist (Catholic or pro-one Ireland) will do the same, and Northern Ireland will return to the days of violence and fear that was the hallmark of the 1970’s and 1980’s.