I’m old enough to remember “The Troubles “, I remember, on holiday, hearing that the McDonalds I and my friends regularly visited, had been bombed.
IMO one of the IRA or the UDA (or whatever their current incarnations call themselves) will find a reason for violence for any border, if it’s between North and South the IRA will kick off, if it’s NI and the mainland the UDA will kick off...
At that point, there will be tit for tats and the whole ungodly mess will start again. I don’t believe that Boris or Rees Mogg are old enough to have had any day to day real understanding of what The Troubles really meant.
Almost any kind of Brexit will seriously damage the economy of NI, and it is a historic fact that when there is an economic downturn the extremists gain support.
I strongly believe that one of the main benefits of the EU is that it has made internecine conflicts irrelevant or has at least toned them down, Ireland, Basque, former Yugoslavia.
I don’t believe that there is a practical solution to the “Irish Problem “ , other than a very soft, and therefore meaningless Brexit.