It's a frictionless border now, and there's no reason not to remain one after Brexit. One may have a frictionless border between any adjacent countries who agree to one. All that is required in this case is for the EU not to be so adamant that there must be a hard border around the EU. They already have soft borders elsewhere, precedents have already been made, so there's no excuse. The intransigence has to be for ulterior motives.
Britain does indeed want to set up its own customs and immigrations regime which differs from the EU, and a soft border can be part of that regime. Full scale border checks won't be needed, merely sufficient monitoring to ensure that the border isn't being abused. It is in the interest of both countries involved to ensure that doesn't happen.
Unfortunately Southern Ireland seem intent on complying with EU demands and because that ensures they are somewhat impotent, they opt to cause issues with the threat of a veto on trade talks. Presumably knowing it raises the chance of causing a "hard" Brexit and a polarisation on positions; basically ensuring a "hard" border. Clearly they either haven't thought things through, or are hoping that threatening trade talks will change minds.
The mind they have to change is the EU's, so their ire would be better directed there. Mind you, we seem to have such a weak negotiating team, who knows, they may get a UK capitulation yet.