What is a hard border anyway? What are the important issues either way?
In terms of movement of people (for holidays at least) you need to show passport if you are travelling from France to either UK or Ireland but not between the Republic and NI. So that doesn’t have to change in principle and that should be enough to solve “protecting the borders“ issue from Brexiter point of view.
In terms of work the current status quo of people in NI essnetually having a choice between UK or Irish or both passports has a lot of importance culturally but little in terms of reality as the right to work and trade either side of border is same regardless.
But what about trade and regulation? What happens in terms of trade if there is a no deal Brexit? What are the risks to both sides of you have no physical checkpoints but with no alignment over policy and regulation. In long term I can see how argument is that Ireland (and hence EU) can get flooded with goods from a different juristriction but how does this currently work given the fact Ireland has different taxation levels.
Do UK authorities just turn a blind eye to it, are there existing border checks or is there a more subtle process?
I'm not sure why you think the passport issue doesn't have to change post Brexit.
Consider removing the freedom of movement thingy - something very high on most Leavers' checklists - how does that mesh with no passports, no borders, no checks? If it doesn't matter, then why have getting rid of freedom of movement as a big thing we must achieve?
As well as human traffic, there are as many issues with taxation, regulation and law for goods and services crossing that border as there are for the borders at the ports. Again, if control and regulation of these areas matter - a reason people want out - they have to be enforced, otherwise, what is point?
Then, on the Irish border issue, you have the unimaginably huge fucking can of worms currently lidded by the
Good Friday/Belfast Agreement. You really need to read about it, read about Britain's conquest and rule of the island of Ireland and the terrible divisions and violence that has sprung directly from it, understand, to a degree at least, Britain's responsibility for it and its responsibility to do
everything in its power to prevent any more bloodshed or anguish. I simply cannot believe that any sane person could start fiddling around with this, it's beyond unacceptable IMO.