BTTW talks of communities burning down the border posts. I don't consider myself particularly "hard-line" or whatever - I'd like to think I'm generally reasonable - but I'd similarly have no issue whatsoever with border communities disrupting the operation of border posts and removing the physical infrastructure if possible, just like how the same communities
removed the British army's divisive and disruptive reinforced concrete blocks from border crossings on "unapproved roads" during the conflict. I'd defend those communities who sought to take such action. Brexit explicitly contravenes the Good Friday Agreement, in which a common EU dimension, upon which much of that agreed is implicitly based, is specifically mentioned, so there's very legitimate anger here.
That's not to say I would condone the threatening or taking of the lives of those staff manning border posts - I wouldn't and would strongly object to that - but I'm sure there are some - "dissident" republicans who have access to explosives and weaponry, perhaps - who would threaten to go further than simply causing infrastructural disruption and damage. All you need is a sniper waiting for an opportunity from the hills or fields around a remote border crossing, most of which are rural. An entire security infrastructure (on top of the customs infrastructure) may well be required to protect against threats such as those I have outlined.