WTO rules are that no nation treats another nation more favourably than the rest, unless both nations have a customs union.
If the UK has no border to prevent goods crossing the border from ROI without tariffs, it will be treating ROI more favourably than the UK, and vice versa. To avoid breaching WTO rules, there needs to be either a border, or a CU.
The ROI is also obliged to comply with WTO rules.
The EU has a duty to all its members to preserve the integrity of the single market and to ensure that goods traded in the EU comply with EU rules. There will have to be a border to prevent non-compliant goods from entering the EU. They don't want to be awash with chlorinated chicken from the US, ffs.
The Good Friday Agreement was settled at a time when Brexit wasn't even thought of. It is entirely dependent on both sides being members of the EU, or having a close trading relationship (eg single market/CU).
The whole border thing wouldn't be an issue if it wasn't for the ERG headbangers, many of whom stand to make a lot of money from the UK leaving the EU, weren't so determined to wreck not just the economy, but also the fragile peace in NI. It probably wouldn't have been an issue if the GBP had paid attention in the run up to the referendum and not been so willing to believe that the border issue was a bit of nonsense made up up by Europhiles and part of some mythic Project Fear.
So, all you Brexit-supporting people, any blood split or lives lost as a result of the outcome of this will be on your hands. And I will remind you of that at every opportunity.
I also think that people who can't remember the horrors of the 70s and 80s need to learn some history. I can remember only too well, and it was terrible, and terrifying.