The creation of a hard border between the northern and southern parts of Ireland raises the possibility of an upsurgence of terrorism. The the shared umbrella of the EU was the support for the invisible border This eased travel and traffic between the two sides. It was not a united Ireland in law, but in some ways de facto, it has appeased all parties and put Sinn Fein in its place.
The creation of a hard border — should it be the result of Brexit — sets the scene for the return of the hard men, who can point to the hard border and make it a fetish for their cause. Things that are hard are unsubtle, they invite harshness. Bertie Ohern has just said the hard men will be drinking the champagne as the British get pushed into the hard border.
Division weakens the position of the peace lovers and gives a cause to the men of violence. Of the many reasons to fear Brexit, this is the gravest. The Brexit idiocy plumbs new depths.