A United Ireland is a very worrying thought.It's far too late to actually build the infrastructure for a hard border. Farmland which straddles the border will need to purchased by either the UK or Irish government. The border checkpoints etc. will also need to be constructed. It would take at least five years - more like ten or fifteen - for border infrastructure to be created.
A new Boundary Commission may need to be put in place - simply because decisions around where the border is will need to be sorted out.
The big difference is that unlike the original Boundary Commission, where Britain was the economic powerhouse and Ireland was a poor country having a civil war, the tables will be turned.
It's also worth considering it's highly likely there will be a nationalist majority in Northern Ireland in the next ten years.
I reckon there will be a general election in the UK and the DUP will find themselves out of favour in Westminster. The makeup of the UK parliament will be totally unlike what's it has been, but I think it's reasonable to assume that English voters won't have any problem with Northern Ireland remaining in the EU (as in cutting it from the UK; or the UK staying, in some shape or form, in the EU).
The North of Ireland will be a mess for the next five years, but I think what's going to emerge is a united Ireland.