Frank uses unpleasant visual imagery when he describes the meager portion of corned beef his mother has managed to bring back for her starving children to eat:
Mam has the fire going and something boiling in a pot. Malachy smiles and says she brought home corned beef and a few spuds from Kathleen O’Connell’s shop. He wouldn’t be so happy if he knew he was the son of a beggar. [...] It isn't corned beef at all. It's a great lump of quivering gray fat and the only sign of corned beef is a little nipple of red meat on top. We stare at that bit of meat and wonder who will get it.
McCourt’s use of imagery here makes the reader feel just how unappetizing this meal is. Even though the McCourt boys are starving, the “lump of quivering gray fat” is still not an appealing prospect. Instead of a nourishing dinner, his mother can only bring them this pitiful, “quivering” offering. It's made worse by the fact that Frank knows she had to beg their local shopkeeper for credit in order to get it. He feels ashamed at being "the son of a beggar" and explains that Malachy would too if he were old enough to understand.
When Frank sees the meat, however, the situation seems even worse. The visual imagery here draws a comparison between the lump of fat and scrap of meat and a mother’s breast. Despite how much she wishes to, Angela feels she's failing as a mother as she cannot keep her children adequately fed. There is almost no real food there at all, just a "nipple of red meat" on top of a lump of fat. The imagery underscores that Angela is desperate to keep her children alive and to nourish them by any means possible.