They fear their complaints could provoke a backlash from major retailers. Those complaints include being asked to pay "hello money" in return for shelf space, and to fund retailers' advertising campaigns, marketing and promotional initiatives, "as well as other costly demands that have nothing to do with suppliers' contracts", according to Paul Kelly, director of suppliers' organisation Food & Drink Industry Ireland.
The suppliers are afraid to speak out on such issues because of the buyer power of the major retailers and the worry that they could be delisted if they are seen to rock the boat, said Kelly.
"It's just the reality of the marketplace. You can't afford to make a stand, especially in the current economic climate, when a substantial part of your business may be at stake."
The parliamentary committee, which is examining why grocery prices in the Republic are so much more expensive than in Northern Ireland and the UK, has heard from the multiples, including Tesco and Lidl, as well as independent grocers' group RGDATA and the Competition Authority. All gave evidence in public.
However, with suppliers refusing to do likewise, the committee is now exploring whether to hear their views in private, by way of written submission or through a detailed list of questions.