The Good Friday Agreement says there won’t be any border checks or post. Apparently if there are the Republicans and Unionists will start blowing things up again.
Actually the Good Friday Agreement is completely silent on the matter of border checks and posts, or any concept of a hard or soft border. Indeed it makes the border very explicit, and recognises the sovereignty and territorial integrity of both the Republic and the UK. But it does have some vague language about the people of NI and the Republic having freedom to conduct their daily lives as if the island was undivided, and this has been made easier by the 'borderless' aspects of the EU, but much more importantly by the reduction of violence, making military checkpoints largely unnecessary.
It is the reduction of violence that has reduced the 'hardness' of border, not the other way around.
But, yes, anything that changes the status quo in Ireland is bound to risk increasing tensions, and while a 'harder' non-EU border between the two might aggravate Republicans in NI who have been pretending they live in the Republic, there is no actual breech of any agreement, or real excuse for any return to violence.
Furthermore, any 'harder' border could easily just about ignore all individual citizens crossing for work, social or shopping reasons, even if there were tariff differentials, since it is not material whether someone buys their beer in Tesco in Newry or Dundalk. And illegal immigrants from the EU can just walk across any field and good luck to them. All that is needed is some checks on commercial border crossings to control the transport of large volumes of goods where there are significant tariff or standards differentials between the UK and the EU (which will be negligible anyway).
The whole thing is an exercise by the EU in making it as difficult as possible for the UK to leave, and we should never have engaged with it.