Tmac,
There are many discussions on the internet on the subject of Brexit and border between the Republic of Ireland and Northern Ireland becoming the EU's external border if the UK leaves the EU. The 'backstop' related to this possibility seems to have been the most contentious 'red line' subject throughout the recent negotiations between the UK and Brussels.
The subject is both contentious and complex, so I don't begin to understand it in any depth at all. Some information is
here
I would rather Ireland did not return to the pre-treaty situation and consider that peace there is far more important than the status of any UK or EU politician, so the sooner that they stop posing and posturing and settle down to agreeing a compromise, the better.