"It's really not that hard to understand!"
I'm afraid it is. The EU may have other borders but the Irish one is the only one that matters here.
"RoI will have to enforce a hard border,"
Ireland has stated categorically that they will not impose a hard border, however the UK leaves the EU. Even if they were so inclined, they simply would not be able to do so. There are over 200 crossing points between the two countries spread over 300 or so miles with huge complications if it were to be policed (which it hasn't been effectively for over a hundred years):
https://www.irishtimes.com/news/ireland/irish-news/ireland-has-208-border-crossings-officials-from-north-and-south-agree-1.3474246
So once again, who is going to do it and how? Simply trotting out the same thing over and over that "something will happen" does not answer the fundamental question.