When I was a kid in the '90s I saw the Ira fighting to make the whole of Ireland separate from Britain. One Ireland that was independent of the kingdom. The loyalists and Ira were constantly fighting a paramilitary civil war. They were even detonating powerful bombs in Britain itself.
During Brexit the border between Northern Ireland and the Republic of Ireland had become the border between the UK and EU because the Republic of Ireland remained a part of the EU trading block.
The problem is the EU member states don't want products and services from low income, low tax countries to enter the EU trading block without being charged a tariff to make the products less competitive with theirs. For example Britain could buy 12 tonnes of olives from Brazil and send them across the Northern Ireland border and undercut the prices of Spanish and Italian olives. The farmer's incomes in Brazil are less than in Spain and Italy.
Apparently our government have negotiated a deal with the EU member states that allows Northern Ireland to have full access to UK and EU markets without tariffs and stipulations. They call it the Windsor Framework. Northern Ireland can sell their products and services to the Republic of Ireland and the Republic of Ireland can do the same with the UK. Northern Ireland has become a unique place on the continent.
Let's see if it works. My local MP is the minister for northern Ireland and he is a Brexiteer, he approves of the new deal. It's a small world.