CTA will remain (free movement between UK and RoI) so long as Dublin keeps out of Schengen. There will be a physical border, as to how hard it will be is dependent on how much regulatory divergence there is between the UK and EU. At the very least, by becoming a third country, there will have to be customs control facilities on the border (SPS checks and the like).
In short, there's going to be a border and the EU needs to accept that so we everyone move on. The idea the EU actually cares the Troubles is a joke, it's just a convenient excuse to get an advantage. They want to use NI as leverage to keep the UK within the EU regulatory sphere.