Their market share in the Republic rose from 1% in the year to 5 October 2008 to 2.4% this year, according to TNS Worldpanel figures.
The figures reflect the soaring popularity of cross-border shopping, as consumers from the Republic travel to stores in Northern Ireland to take advantage of cheaper prices.
Over the past year the price difference in stores on either side of the border has increased, caused by the weakening of the pound against the euro and the reduction in UK VAT.
The TNS data also revealed Tesco's market share had fallen from 26.3% to 25.9% over the past year, but increased from 26.1% to 26.5% over the past 12 weeks, following the launch of a high-profile price-cutting campaign in May to stem the flow of cross-border shoppers.
Lidl and Aldi have strengthened their position, increasing their combined share from 7.5% to 8.7% over the year.
Last week, Nielsen revealed some 250,000 households in the Republic now shop regularly for groceries north of the border a figure 25% higher than last year.
The news comes as Asda revealed that it would add an extra 150 spaces in the car park of its store in Enniskillen, Northern Ireland, to cope with an expected Christmas rush.