Re: "There are over 200 roads that cross the border."
The N1 South of Newry, the Northbound carriageway crosses into the UK for 10 meters.
About 100 meters to the west - Church Hill B113, the border goes along the road.
Another 3km west - Kilnasaggard Road, the border again goes along the road for about 1km
I'm not going to examine the rest of the border, but you get the idea.