I'm just wondering how great is this threat.
Do you want to find out?
It's not like there's a specific threat in place for Day 1, that will see X killed in NI and Y killed in GB (although, fuck knows, maybe there is).
Introducing a hard border will have a massively negative impact on people's lives. They won't be able to do things they did before, businesses will suffer, jobs will be lost. The first people hit by this will be those with no buffer - as ever, economic strife comes first for the poor, not the rich.
As people see every facet of their lives changed, they will get desperate. This is becomes a rich recruiting ground for paramilitaries. As attacks on border posts ramp up, so do retaliatory attacks on nationalist communities.
People begin to distrust the people around them, especially those from the other community.
Then, that's it. That's the Troubles again. Instead of rubbing along happily enough together, the old divide becomes apparent. Security is stepped up, and then the security is attacked. Old wounds reopen.
At the minute, there are still paramilitaries but they don't have support within their communities. People have seen peace and they like it. But trust me when I say, massive disruption to day-to-day life and huge economic penalties will whip us back to the bad old times quicker than we'd like to imagine.