Shoppers from the Republic account for half the weekly 45,000 transactions at a single supermarket in Northern Ireland.
And it has emerged that most of the customers who cross the border to the Asda store in Strabane, Co Tyrone, hail from Co Donegal.
Last Tuesday, when public service workers went out on strike, trade doubled in places like the Foyleside Shopping Centre in Londonderry, where almost 30% of cars that day were from the Republic.
But even without that boost stores in border towns from Newry in the east to Strabane in the west have been enjoying a steady rise in cross-border business.
One retail analyst at the University of Ulster confidently predicted that by the end of 2009 the loss of revenue to the Irish economy could exceed €1 billion.
Nowhere is the unstoppable trend more evident than in the spanking new 52,000 square feet of retail space occupied by Asda in Strabane.
On Friday evening the 28 tills were ringing to the sound of euros, which have helped make it the second most successful Asda store in Northern Ireland after Enniskillen.