There will be a ‘hard’ border with border crossings and checks between Ireland and Northern Ireland after Brexit. The UK want to keep the European hordes out, and the Europeans want to preserve their customs union. It is basically unavoidable.
We can’t say we are a special case. The EU has thousands of miles of external borders and each one of them has a sad story. We can’t expect special treatment for Northern Ireland, or if we do, Ireland is going to have to pay a significant price for it.
Even the most ardent Brexiteer will admit that the first years of Brexit will be economically tough for Britain. As a result, Northern Ireland will see the current subsidies from London cut brutally.
Northern Ireland will be a pretty terrible place to make a living as a result of budget cuts and border curtailment. No one will want to invest in factories or other productive facilities in the North. Trade in goods will be very limited with the South, though there will be plenty people commuting across the border for work in the South. It will be like a big dormitory community.