The Irony of Bullying
Bullies often take out aggression on their victims because they are projecting how helpless they really feel. In order to regain some semblance of control over themselves, they try to make somebody else feel worse. It's a power move. In Crash's case, he desperately wants approval. When he doesn't feel like he's getting the attention which he deserves, Crash responds by withholding approval from Penn. He's the one with the problem, not Penn.
The Irony of Mike's Absence
Crash is friends with Mike because he thinks he's cool. He appreciates the approval of somebody popular, but when times get tough Mike isn't invested enough in the friendship to stick around. He leaves Crash to sort through his own problems. In response Crash feels lonelier than ever before because now he realizes that that friendship was meaningless the entire time. He could've been friends with somebody like Penn who really cared, but he chose the flashier, more prestigious option which backfired.
The Irony of Materialism
The entire time that they're growing up, Crash doesn't accept Penn because he thinks he's weird. When they first meet, Penn only has two toys, one of which is a tin of dirt. He just leads a different life, not based upon materialism and popularity. Crash chooses to go the opposite direction, seeking status symbols and the envy which they elicit in his classmates. He regrets his choices, however, when he starts exploring his soul at the prospect of his grandpa's death (reminders of death are tricky like that). He would have been better off all these years valuing knowledge and experience like Penn instead of status and objects.
The Irony of Penn Winning the Race
Penn is not a competitive spirit by any means, but his hero -- his great-grandpa -- is. He wants to make his great-grandpa, his namesake, proud by winning the big race at school. In the end, Crash, who really does care about winning for the sake of competition, throws the race in order to let Penn win. Penn, the kid who never cares about winning, finds himself achieving more than he ever dreamed not for his own sake, but for his great-grandpa whom he loves.
The Irony of the Tin Can
Penn's favorite possession is a tin can full of dirt, which is dried mud from the Missouri River which his great-grandpa gave to him. To Crash this is an odd toy. It's not a toy at all, just dirt in a can, but Penn loves it. He values it because his great-grandpa gave it to him. In this way the object of the can is meaningless but his possession of it is proof of a mutual respect between him and his ancestor. The dirt can become anything with a little bit of imagination, so in a sense his great-grandpa gave him inspiration, a good will blessing.
