If you dont have a hard border there would be no regulation of trading between one economic block and the other. Again, the likes of Newry or Dundalk would be economically wiped out depending on which market is stronger. This would happen to towns all along the border. The smuggling and blackmarket rackets would actually be far easier than with a hard border. This would lead to a huge increase in social issues. You could quite easily have a situation where the UK has far cheaper prices for goods than the EU, this economically cripples Dundalk, Clones, Sligo, Letterkenny etc. As these towns hit economic hard times, they become awash with drugs and anti social behavior as the easiest way to make money is on the black market.
So yea, an open border, the way it is, between two economic blocks would be disastrous for one side at any given time.