I could be wrong but it seems like one of the tough things about this issue is that the border isn't just a practical thing, it's a symbolic and emotional thing.
Rationally, the RoI and Northern Ireland ARE different countries. And if Northern Ireland leaves with the rest of the UK, it makes sense to put a hard border across there. It is, after all, the point where an EU nation meets a non-EU nation. Fact. (I think a lot of Leavers seem to take this position).
But the problem is that it is this very division that is the bone of contention in the Troubles. So having a visible symbol of the de facto division on the border is likely to lead to violence. It's almost like people can let the division be if there isn't something physically present, but if there is, it has to be noticed and reacted to. The pragmatic reality is that a hard physical border will lead to injury and death. And this is not something that can simply be solved by 'locking everyone up'!!
The only real, practical solution I can see is for the UK to stay in the single market and customs union. But that's absolutely not what the Leavers want to do. And to be honest, many of them in England, Scotland and Wales don't seem to give a flying fuck about Ireland. They're all "WHY CAN'T WE JUST LEAAAAVE!" So we find ourselves at stalemate.