The irony of failure and need
Failure is literally ironic to the guests who are blindsided when Seymour doesn't show up to his own wedding. It's also ironic because the reader learns that Seymour is afflicted by mental depression, which the wedding party didn't even take into consideration. The implication of this is that ironically, sometimes we judge people who are serious need.
The irony of Buddy in the car
Buddy never tells the wedding party that he is Seymour's brother (dramatic irony), which allows them a moment to demonstrate the integrity (or lack thereof). Confident that no one will tell the bridegroom's family, the bride's wedding party badmouths Seymour, assassinating his character. Then, Buddy finally reveals his identity.
The sad irony of the Introduction
We meet Seymour in the second story of the novel, an intimate stream of consciousness in which Buddy teaches the reader why he honors his brother's legacy, in spite of a community that often judged the suicidally depressed (many believed during that time that suicide meant someone's soul when to hell). The irony of the introduction is that it actually serves as a defense, not an introduction, because the person we're "meeting" has already died. We are being invited to a legacy that we are either to celebrate or reject.
The irony of the poem
The poem from the title of the first story shows up when Boo Boo leaves a kind note for her brother Seymour on his wedding day, putting the line on his bathroom mirror. The note is a nice picture of how grand and powerful the groomsbride is, a thoughtful gesture for Seymour's wedding day. But it was on the mirror, to help his self esteem, because he was supposed to see it and associate it to himself. The irony should be rather obvious in that Seymour doesn't actually arrive at his wedding. Perhaps the obstacle was his self esteem, as the location of Boo Boo's note on the mirror suggests.
The irony of commitment
In some ways, Seymour backed out of his wedding at the last possible moment he could without breaking a literal oath. It's ironic that everyone is mad at Seymour, because ultimately, he did the right thing by not committing himself half-heartedly. In other words, Seymour is rejecting some of his "social responsibilities" because he has business to attend to within himself. In this case, it might be the wise thing to do to not get married.
