They'll break your legs or knees for something as simple as anti-social behaviour in certain areas. Maghaberry though is just up the road from Maghaberry HMP, so most of the populace and surrounding towns are cops and prison staff (at least from my experience living in the area ten or more years ago). But there's always someone that knows someone (again, just my experience of the area).
But as I said, if even the threat of paramilitary justice doesn't make someone stop and think twice about what they're going to do, 4chan isn't a problem.
To be honest, I'm surprised it happened in Maghaberry. It's a nice wee quiet place. If the headlines had been "dog set on fire in Lurgan/Lisburn" I wouldn't have been all that surprised. But there's the chance that these two individuals, if they were at a house party, have come from outside the Maghaberry area.