One of the most common forms of situational irony in literature is when children take on traditional adult roles. This is ironic, of course, because young children are expected to be dependent on the care, experience and wisdom of the adults in their lives.
This is the ironic premise that The Glass Castle is built on. The main character, Jeanette, and her 3 siblings, are subjected to what should be considered abuse and neglect by their parents over and over again as young children. Most often they are taking care of their poor and crazy parents, making excuses for the lack of parenting, and watching out for one another. However, throughout the kids' lives, they never question whether this is normal. Likely they know they are different from other children - but never do they consider their lives as bad. In fact (further irony) the story is lightheartedly tragic as they celebrate what would otherwise be considered child abuse as "adventures." Also, throughout the story, all the children love each other and their parents more than anything in the world. I think too that through such an unconventional upbringing, the children eventually consider themselves better as a result.
If you keep this in mind as the foundation for irony in the novel, you can find specific examples to support it in absolutely every chapter.
A specific example of irony in The Glass Castle is when Jeanette and her siblings decide to "help" their dad begin building the glass castle that he has promised them. When the Walls family moves to the dismal Welch, West Virginia, the children innately search for ways to make things better. Believing that their father truly wants to build a dream house for them, Jeanette and her siblings decide that if they start by digging a hole for the foundation that their dad will be inspired to begin building the "castle." Sadly, their dad does not take note of their efforts but begins filling the hole with the family's trash. His act is symbolic and ironic. The children have faith in their father and his imaginative promises, but those promises turn out to be nothing but trash literally and figuratively.