The irony of maternal abandonment
Maternal abandonment is especially ironic, because the bonds of nature are tighter between a mother and son than a father and son. This is technically the case in two ways, because women carry babies through pregnancy and deliver them, and because women breast feed their young and ween them. So, the real implications of motherhood stand in stark contrast with a mother willingly abandoning her son to whatever fate might befall him. It's the opposite of motherly.
The cold nun
So opposite of motherly is Jennings' mother that when Jennings arrives at the care of the stern nun, Sister Frances. Her no-nonsense approach to responsibility and her authoritarian demeanor are types of womanhood the boy has never seen in his mother, so the nun represents something very particular—she represents the opposite of his mother. On her surface she seems cold, but secretly, she is very warm and charming with a tough front. This is ironic, because Jennings' mom is accepting on the front, but when no one is looking, she is a traitor of sorts.
The false rescue
When the Carpenters are discovered for their bad parenting, the worker finds a badly beaten orphan child who was fostered by them only for monetary gain, it turns out. This ironic fate means they are false rescuers. Instead of accepting the role of foster parent seriously, they do it for the financial gain and they neglect the child, the same problem from before.
The ironic return home
What is more ironic than a long line of foster homes and orphanages? To return to the house of the mother who abandoned him. But ironically, that's exactly what happens. But this time, it's perfectly obvious to him that his mother is incompetent. He struggles with severe negative emotion, unsurprisingly, and he realizes this 'home' was never truly home to him.
The irony of homelessness
Even when he has a home, Jennings is homeless in a way. His mother and father have taught him that he should esteem himself lowly, and they have abandoned him. The damage that causes his psyche prevents him from being able to connect well with people, and besides that, most people are just exploiting his situation instead of really trying to help him. Ironically, he isn't at home until Sal decides to personally handle the situation by fostering Jennings himself.
