@Anlaf
And even this is not totally right.
There was a reasonably detailed Northern Ireland Affairs Select Ctee report in May 2016, i.e. before the referendum:
However, in the event of a Brexit, the border between Northern Ireland and the Republic would become an external border for the EU and so would principally be a matter of negotiation between the UK and EU.
Not really. It's binary. We're in SM/CU or not.
The scope of any post-Brexit trade deal would have an influence on the nature of the border.
No. ANY trade deal requires a border. SM/CU membership does not.
In the event that it did not extend to mutual tariff-free access to each others’ markets, the border might need to include customs checks.
If it were just tariffs that would be fine. But it's not. Regulatory compliance!
Furthermore, if any post-Brexit agreement between the UK and EU did not extend to the free movement of labour with the rest of the EU, there are fears that the border with the Republic would become a “back door” by which UK border controls could be evaded
Love how Brexiters are saying it is not they who want a border but the EU.
Even under bare bones WTO, the UK MUST have border controls.