There isn't one - a backstop. It's an agreement that they can't and probably won't ever make.
And I do care. Although I'm Irish, I live in England now, and anticipate living here for many more years. I care because I believe that the UK needs to sort out its attitude towards NI. They want it but most don't give a shit about it. It's a rare person on this island who can tell you where the border is, what the government is or believes, what the Good Friday agreement is, what abortion law is there, or indeed anything about the place. Which wouldn't matter, except that about half the people there want very much for the rest of the UK to pay attention and remember that they are part of the UK, and should be treated as such.
As an Irish person, I have a vague interest in a united Ireland, but the UK has ignored the North for so long, and let its economy stagnate so badly that I don't think we could afford to run it right now anyway. I don't mean any disrespect to the the people of Northern Ireland when I say that - I mean that they have been ignored and treated as a "problem" for too long, whatever way they vote.
The Brexit issue is at the very least reminding people in this country that Northern Ireland exists and needs to be taken seriously. They voted to remain and instead, are being dragged out of the EU, and into another border row.