Regarding DD's clarification, seems like it is a gentleman's agreement with no legal binding after all. And of course it's been an insistence of the EU that either everything is agreed or nothing is.
http://www.bbc.co.uk/news/uk-politics-42303059
But since the UK want a soft border it can still be done in the event of no deal. Eire merely have to agree it with the EU, because they've already a gentlemen's agreement with the UK.
Based on the agreements reached so far by the negotiating team, I'd guess many people, NJ included, are smarter than everyone in the negotiating teams on both sides put together. But of course, both sides are not working towards a fair solution, one in particular is trying to punish the other for having the audacity to opt out. Despite the possible loss to themselves of causing poor relations.
As for the scenario whereby the Republic of Ireland decides to join the Schengen area despite having seen what an utter shambles it is, then we'd rely of the tech to monitor those coming down the roads. No one can monitor a whole borderline well regardless, so it's not like anyone has a better solution.