Has the Newry effect begun to wear off? Research by TNS Worldpanel would suggest not as Asda and Sainsbury's saw their share of the grocery market in the Republic rise by a third to just 3% in the fourth quarter of last year, compared to 2.2% last year.

The amount of penetration they have into the southern market has also increased sharply, up to 20% from 13%.

By contrast, however, shoppers from the Republic are going to the two British supermarket giants less regularly, visiting an average of just under four times over the three months at the end of 2008, compared to an average of 3.4 trips in the same period last year.

The amount spent by a shopper from the Republic in the shops in the North dropped slightly to €64, compared to €61 the previous year.

That reflects the fact that customers stock up when they travel north. The average total grocery spend per visit in Tesco and Dunnes by contrast is €32 in both cases.

Taken over three months however, the multiples with stores in the Republic win hands down.

The total spend by shoppers from the Republic in the North fell from €248 to €203; the spend by Tesco customers over the three months fell from €468 to €441 the previous year, while the average at Dunnes dipped from €516 to €444.

SuperValu's customers spent an average of €443 over the quarter, compared to €487 on the same period in 2008, while the average spend over the three months at the discounters was €159, a rise of €1 on last year.

Asda is currently planning to launch a home-delivery grocery service to the Republic, although its asda.ie website routes customers to the UK site at present. Some of its shops in Northern Ireland generate 50% of their sales in euro.

Rumours about a possible takeover of Dunnes circulated again in recent weeks when the Irish supermarket chain allegedly circulated a requirement for a cherry picker to change store signs. However, a number of retails sources said the rumours were incorrect.