DMcG, there are hundreds of roads crossing the border, mostly remote B roads and country lanes.
The border always has been porous and always will, but technology will make it harder for the crooks to get away with it.

BBC 2001,
"During 2001 customs officers in Northern Ireland seized thousands of gallons of smuggled diesel, millions of smuggled cigarettes, bootleg liquor, fake CDs, DVDs, computer games and football team merchandise. "