When I ask people their biggest concerns about “Brexit”, the common response in Northern Ireland is “the border”.
An important, brief point here: in the negotiations between the UK Government and European Council, the border will not feature at all.
So, if you hear anyone telling you what they intend for the border, beware! What happens to the border will be determined by the law of unintended consequences.
The key aspects of the negotiations which are relevant will be free movement – of goods and services (primarily covered by the Customs Union), of people (primarily covered by the Common Travel Area) and of labour (primarily covered by the Single Market, or “EEA”).
If the outcome of those negotiations results in anything short of free movement on all of those counts, then the border will need to be manned – quite obviously. Again, beware anyone who suggests otherwise!
If the UK leaves the European Union Customs Union, then there will need to be some sort of customs control at the border (unless the UK maintains exactly the same Customs arrangements as the European Union – something which would surely defeat the object of leaving it). That need not necessarily be immediately at the border itself; it could be well to either side of it (and indeed it could be so far north of the border that it effectively takes place at the ports).
If the UK leaves the Common Travel Area (probably consequent to leaving the Customs Union), then there will be passport checks. This, it has to be said, is very unlikely; furthermore, even if it did happen, it could even be managed in such a way that the checks take place at ports and airports.
If the UK leaves the Single Market, then again there will be tariffs on goods going in either direction which will mean border checks to apply them. It would be no use Northern Ireland remaining within the Single Market, because then it would face tariffs with Great Britain. Again, the scale of these checks will depend on what precisely tariffs are applied to.
The key issue here is that what happens to the Border will be determined by other parts of the negotiation. It is not, itself, part of the negotiation.