Brexit from the EU by the UK in a couple of weeks time, will in my view fairly soon thereafter, bring to a head the need for a referendum in Northern Ireland about the six counties unifying with the Republic of Ireland.
The Euro is already in fairly common use in the North and after the UK is out of the EU, trade between the six counties and the Republic which of course already uses the Euro as its national currency, will as I understand it be unaffected by Brexit.
However, after Brexit, trade between Great Britain and Northern Ireland will in theory at least be subject to some kind of customs checks so logically the people of the six counties should soon consider whether their best interests would be served by reunification with the remainder of Ireland. Passport checks have not proved an issue for travel between Great Britain and Ireland in the past so should not do so in the future.
Monetary cost is of course not the principal reason for being part of one nationality or another, for example the European Union itself, but is factor.
The cost to Great Britain of supporting Northern Ireland annually, exceeds the saving in annual costs likely to result from Brexit.
My own belief is that the reunification of Ireland would be good for Ireland good for Great Britain and good for the EU.
Hopefully the time will come fairly soon, for a referendum in Northern Ireland about rejoining the South.