The irony of Arnie's attention-seeking
When Arnie acts out in dangerous ways, he doesn't perceive the threat of harm very well, but he does understand that when he does what he's not supposed to, people suddenly pay him attention, so whenever Gilbert is not paying him enough attention, he threatens his own safety to get what he wants. This is ironic, because Arnie is putting himself in serious danger every time he acts out—he often climbs tall towers because he likes to gather a crowd.
The irony of Ellen's innocence
Ellen is disturbed and depressed, just like everyone else, but as the youngest in the family, she demonstrates the least innocence as a character. She wears very erotic makeup and clothing, but she's just barely a teenager, so it's obviously rebellion and bitterness that have matured her. That means that underneath all of the anger and experience, Ellen is still just mourning the original loss of innocence that her father's suicide meant. The irony is that she is the youngest sibling, but also most overtly sexual and outspoken.
The irony of the anti-mother
Grief turned the Grape family mother from a warm, hospitable caretaker to a black hole of need. She constantly needs more food (which is ironic, because mothers typically represent provision and nourishment, like a mother breast-feeding her child). She constantly loses her temper (like many in the family do), and she is obviously still consumed with horror and grief, but that means that her kids are left without their mother. She is more of a dependent than a parent.
The irony of Gilbert's betrayal
Gilbert loses his temper at Arnie because Gilbert humiliated himself to replace Arnie's cake, which Amy had painstakingly made, so Gilbert is sad for his sister, angry at circumstance (or even God perhaps), and Arnie is being Arnie. Arnie knows Gilbert is upset, so he does something bad to get Gilbert's attention, but Gilbert snaps and betrays his one and only commitment in life—to protect Arnie. They work through this incident, but even afterward, the irony is painful and obvious.
The irony of the mother's death
Here are three ways that the mother's death is ironic: First it is ironic, because Arnie was the one wasn't supposed to make it to 10 years old. The whole novel is basically about whether Arnie will survive to see his 18th birthday, and then when he turns 18, it's his mother who dies, not him. Secondly, it's ironic because the mother climbs the stairs and sleeps where she's supposed too—a feat she has not accomplished in years because of her obesity. Thirdly, they burn they house down, rather than attract a rude crowd who will disrespect her legacy when they remove her body. Additionally, it's ironic that the book starts and ends with the deaths of the Grape father and mother, respectively. Lastly, it's ironic because the death of the mother is simultaneous the end of a grief season, and the beginning of a new one. The children are both heartbroken and relieved.
