One character who is easily identifiable by class based on the criteria you've outlined here is Paddy Clohessy, Frank's childhood friend.

Paddy is seven years old, one of six children, a typical size for an Irish-Catholic family in the 30s and 40s. He is practically emaciated, dressed in rags, usually dirty understandably does poorly in school.

Here is a wonderful passage that illustrates the abject poverty of Paddy's life:

Paddy Clohessy has no shoe on his foot, his mother shaves his head to keep the lice away, his eyes are red, his nose always snotty. The sores on his kneecaps never heal because he picks the scabs and puts them in his mouth. His clothes are rags he has to share with his six brothers and a sister and when he comes to school with a bloody nose or a black eye you know he had to fight over the clothes that morning. He hates school. He's seven going on eight, the biggest and oldest boy in the class, and he can't wait to grow up and join the English army and go to India where it's nice and warm and he'll live in a tent with a dark girl with the red dot on her forehead and he'll be lying there eating figs, that's what they eat in India, figs, and she'll cook the curry day and night and plonk on a ukelele and when he has enough money he'll send for the whole family and they'll all live in the tent especially his poor father who's at home coughing up great gobs of blood because of the consumption. When my mother sees Paddy on the street she says, Wisha, look at that poor child. He's a skeleton with rags and if they were making a film about the famine they'd surely put him in the middle of it.