Consumers who do their shopping in local convenience stores are paying up to 20 per cent more than they would in supermarkets, according to a price survey by the new National Consumer Agency (NCA).
Shoppers in Dublin are paying up to 10 per cent more than they would outside the capital, the survey of leading branded products also found.
However, there was little difference in the price charged by the three main multiples - Tesco, Dunnes Stores and Superquinn - and SuperValu.
While Dunnes emerged as the cheapest supermarket, the difference between it and Tesco and Superquinn for a basket of 45 branded items was only €2.20, or 1.6 per cent.
"This survey shows that if consumers want to save money they are much better off doing their weekly shop in large multiples or SuperValu and then using convenience stores for the odd top-up," said NCA acting chief executive Ann Fitzgerald.
Ms Fitzgerald said it was worrying that there was such a small difference in price between the three main supermarkets. Of the 45 branded items that were common, 21 carried the same price in all three outlets.
"They're either all watching each other and following each other's prices, or there is the potential for a lack of competition."
Suggesting the groceries market is too concentrated, she said she looked forward to a review of the sector by the Competition Authority, due out this year.
Ms Fitzgerald said it might be in consumers' interest for a major outside player to enter the Irish market, such as Asda, the British chain owned by US giant Wal-Mart.
She also questioned why Tesco, which has the largest market share of the Irish market and is backed by a giant UK parent, could not use its greater purchasing power to charge lower prices.
Rgdata, which represents grocers and includes SuperValu, Spar and Centra stores among its members, said the survey showed the larger independent stores were competing "head-to-head" with the multiples.
Its director, Tara Buckley, said it was regrettable that fresh fruit, vegetable and meat were not included in the survey, and claimed these goods are often cheaper in independent stores.
Comparing prices in convenience stores was of limited value, she claimed, because of their "different operating formats and consumer shopping preferences".
In the multiples, there were only three products where there was a 20 per cent or more price difference, with 11 products having a price differential of 5 per cent or more. A basket of 45 items cost €135.61 in Dunnes, €137.56 in Tesco and €137.81 in Superquinn.
However, when the survey was extended to include the so-called symbol groups (mostly convenience stores), Eurospar was 7.4 per cent more expensive than the multiples, Centra was 10.8 per cent more expensive and Spar was 20.2 per cent more expensive.
A 454g packet of Kerrygold butter, for example, cost €1.84 in the three multiples, but €2.79 in Spar Dublin.
Prices in Dublin were just 0.5 per cent dearer in SuperValu but the margin between capital and country rose to 4.6 per cent in Centra shops and 10.3 per cent in Spar shops.