The most obvious example of irony is that the father may dream of glass castles and create magical stories about them, but they actually live condiitons of the extreme opposite of castles. At two different points in the story they live in an abandoned boxcar and later they live in a broken down shack that doesn't have a solid roof, running water or proper heating. The children live in abject poverty. Another aspect of the irony of this is that glass castles aren't real and suggest fragility, which is a clear description of how fragile and vulnerable the children are in the conditions in which the parents have them live.
Another example of irony is that Rosemary, the mother, actually grew up with great stability and a great sense of economic security, yet she allows her husband to make grave decisions for the family and she goes along with them even with the risks to her children's welfare. Rosemary has a teaching degree, but rarely uses it to better the family's life even when she has the opportunity to do so. She seems rather selfish and at times self-absorbed; she behaves in a very contrary manner to the way she was raised.
Other smaller examples of irony: the children have nothing to eat, and the father drinks rather than feed his family. Jeannette has every reason to hate her parents for the way she was raised, yet the novel speaks to the love she will always have for them. The youngest sister, Maureen probably had it the easiest as a child, and yet she is the one who has had the hardest time making herself into a successful adult.