I'm worried about troubles starting again too. That is my biggest fear too, from the other side of the border. I still cannot believe that Parliament voted for breaking international law. Cannot believe it.
Prices will rise, I think. Any border delays, VAT, duties, additional checks, additional compliance requirements etc will all increase prices.
If you have a UK driving licence, I believe you need to get an Irish one to drive in ROI after 31/12/20.
Buying from the UK, if you are in ROI, will be subject to customs charges.
I think businesses will adapt. It will be a big loss of market if ROI can't sell to UK, but UK will still need to import food and ROI is close and exports a lot. And if ROI does export less to UK (quite possible), businesses will find new markets or products. So many new products out now that were unthinkable 12 months ago (covid related), businesses always have to adapt and change.
It's all a bit of a mess, really. Still can't see any upside to it. I do feel like there is no reasoning with the current UK government so we just have to let them off, and work with the rest of the EU. Sorry that's no help to NI. Wasn't it the DUP supported breaking the GFA? There is no reasoning with them either.