Is it a hard border if nationals of an entirely separate country, ROI, are to be allowed to cross it unchecked?
Well, yes.
Just like it was between ROI & UK prior to EU free movement.
The mainland (shorthand) UK has a natural hard border in it being an island.
As an independent country, the UK can decide who it wants to let in unchecked, minimal checks, or otherwise.
If a post-Brexit UK decides that free movement is desirable between ROI & the rest of UK (NI and mainland), then that's the prerogative of the UK and no one else.
If the ROI decide that they don't want that, then that's a whole other issue - as it is their prerogative to decide their own movement controls and borders.
Basically, it's between the ROI, NI & Westminster to come to agreement & decide what the best way forward is.