The outcome of Jake's decision
Readers know that the outcome of Jake's decision to travel back in time and stop the assassination of President Kennedy will be negative. However, none of the characters in the novel know this.
Kennedy's demeanor and values
John Kennedy's demeanor and values, which were generally calm, reassuring, and opposed to war, resulted in a nuclear war that killed countless people. That result, which is incredibly unexpected given who Kennedy was, is an example of irony.
Sarcasm
The quote "sarcastic people tend to be marshmallows underneath the armor" is rich in irony. Sarcastic people are generally rude, hardened, and have difficulty developing relationships with people. Nevertheless, despite this, Jake calls them "marshmallows."
Al's cancer
Al develops terminal lung cancer overnight despite being perfectly fine the day before. That cancer is a byproduct of his traveling through the time travel portal, ironically meaning that, in order to save a life, someone will have to die.
Lee Harvey Oswald
Lee Harvey Oswald, Kennedy's would-be assassin, decides that he will kill President Kennedy despite having no reason to do so. Typically, people—no matter how depraved they are—have a reason or motivation for killing another person. Oswald ironically does not, highlighting his severe mental illness.
