I'm no expert on these matters, but it seems that the solution lies pretty much in changing as little as practically possible. There seem to be many Remainers and indeed the EU citing the border as an issue, but just as no one seems precise about a solution no one seems clear on the problem either. Neither NI, Westminster, Ireland or the EU are willing to establish a hard border therefore it won't happen. I'd suggest some 'welcome to the EU' signage and name seasonal floral displays, much as one sees heading north on the M6 into Scotland, or on the M4 into Wales. It seems a porous border is in everyone's interest, and no one has the will or resources to establish a hard one so I'll leave you to draw your own conclusions