New figures show two Irish retailers experienced a year-on-year decline in grocery sales.
Kantar says the data, for the 12 weeks to July 14th 2019, shows this is the first time this has happened since March 2017.
Tesco sales dropped 0.8% in the most recent period and SuperValu was down 0.5%.
However, overall sales are up 2.7% on last year.
This marks a drop in growth compared with the 3.1% increase experienced 12 months ago - which was thanks to a bumper summer featuring high temperatures and the men's Football World Cup.
Meanwhile, Dunnes is the only one of the three traditional retailers to record year-on-year growth this period, with sales up 6.3%.
While the average price consumers are paying has risen across the board, Lidl has seen the greatest increase as shoppers have bought more of its premium products.
Both Dunnes and Lidl have capitalised on a growing trend among shoppers, who prefer to prepare meals from scratch.
Kantar says recent package deals on the constituent ingredients for home-cooked classics like spaghetti carbonara and bolognese have been popular at both retailers.
Sales of those key meal ingredients recorded strong growth across all the retailers - with herbs up by 26%, spaghetti by 10%, passata by 15% and mince by 11%.
Aldi announced another two fresh produce-focused concept store openings in Ireland this month - known as 'Project Fresh'.
Sales at the retailer are up 12.8% on last year, with spend on fresh products typically outperforming overall growth.
For example, dairy products are up by more than one-third, while vegetable sales have grown by 15% as Aldi continues to capture share in the sector.
Douglas Faughnan, consumer insight director at Kantar, says: "Shoppers swayed towards shorter and more frequent grocery trips during last year's good weather to the benefit of Tesco and SuperValu, which typically boast higher levels of shopper frequency.
"But every silver lining has a cloud, and both grocers were always going to face an uphill battle to replicate that performance this year.
"The retailers have come out fighting in response.
"Tesco recently reintroduced its voucher campaign offering €10 off every €50 spent, and will be hoping a renewed focus on this type of promotion will help it capture a greater share of high value trips and move back into growth.
"The average value of a shop at Tesco is currently €24, behind Aldi on €27 and some way off Dunnes, which offers a similar voucher deal, on €42."