Explain the irony in the following quotation: Myrtle Wilson, describing the thrill of her first meeting with Tom

Buchanan: "All I kept thinking about over and over was 'You can't live forever, you can't live forever.'"

Myrtle it using the terms "you can't live forever" in two ways, one being the way that you feel immortal (through actions that you commit) and the other facing the reality that you can't live forever even if you wanted to.

It's ironic because of this, you can feel mortal but you aren't, its more empathetic than humorous.