Dramatic Irony of the two Wills
When the Will Graysons meet because a cashier calls for Will Grayson the characters are surprised. However, because the reader already knows that the story is about two boys named Will, this scenario creates another layer of tension.
The Irony of Moira
Moira catfishes Will under the name Isaac, hoping that because she knows he's gay, the two of them will become closer friends; instead, this act completely destroys their relationship.
Irony in Love
In the beginning, Green's Will wants nothing to do with love or romance, while Levithan's Will thinks that the best part of his life is his romantic relationship with Isaac. By the end of the book this has flipped and Will 1 has his dream relationship, and Will 2 ends up alone.
Irony in Bonding
Will and Isaac primarily bonded over the fact that they were both depressed, but talking to each other actually made the both of them less depressed.
Irony in Will #2's relationships
Out of all the characters, Will #2 is the one that needs the most support, but he consistently alienates himself from the people reaching out to support him.