i.e. each party says "you do it, I won't", and it ends up that no one did?
https://www.youtube.com/watch?v=J1Yv24cM2os
If you are unaware of the issues with ROI and NI, I'll say that a bit of education is really needed. The history there is very complicated and can't really be boiled down here on HN.
That said, the threat of renewed violence on Ireland is absolutely real and very possible. During The Troubles (lasting ~35 years) there were ~50k casualties in total. The peace is fragile and renewed violence could very possibly result in much higher casualty numbers.
Lives are quite literally on the line.
EDIT: Sometimes pictures are better than words. We can talk about 50k casualties, but seeing some of them is a lot more 'real', at least to me. If you have the inclination, this image gallery does an alright job showing the long and very bloody history of ROI-UK relations during The Troubles: https://allthatsinteresting.com/the-troubles#31
The problem is that by using a trade deal the status quo the UK is effectively losing its voice in the EU parliament without gainign anything in exchange. This is unsatisfactory to both the leave and remain camps of the UK parliament.
There's basically two big stumbling blocks:
The first is that there has to be a customs check between either: (1) The EU/Republic of Ireland , (2) ROI and Northern Ireland, or (3) Northern Ireland and Britain (the island).
(1) is unacceptable to the EU. (2) breaks the Good Friday agreement and could reignite violence in Ireland. (3) is unacceptable to the Northern Ireland political party propping up the government.
The second is that from the perspective of the EU, Britain has to get a worse deal by leaving the EU than what they had when they were in it. Otherwise, every other country sees that it's better to be out than in and the EU risks collapse.
These are more or less intractable problems. I'm sure we've all had the experience where an IT project is obviously doomed but nobody wants the responsibility and blame for pulling the plug. That's where Brexit is today.