I went into Monaghan several times.
The small roads that crossed the border would be blocked by a tank full of concrete. The engineers would place the tank on the border and I would come along with a Puma with a skip full of wet concrete. When they were on the border line I would be hovering over the tank whilst it was being emptied. Most of the fuselage would be in Northern Ireland but the cockpit would be in Eire.
The Garda watching the proceedings didn't seem to worry.