Irony of Arctor's Surveillance
Working as a narcotics operative, Arctor wears a scramble suit while interacting with his government superiors as "Fred," so they don't know his true identity. Midway through the novel, his superior gives him a new surveillance target: a suspicious character named Bob Arctor. This irony is obvious and humorous: Bob has been asked to spy on himself. The deeper, more worrying irony, however, is that Fred and Bob seem to be growing into two separate men; by the end, it's not even clear that Fred sees Arctor as the same man. Both of these ironies play into the book's feelings of comical absurdity and despair.
Irony of Donna
Donna is a dealer of Substance D, and Arctor has been getting close to her in order to follow the chain up and catch her suppliers. Ironically, in doing so, he falls in love with her, and she is considered his "girlfriend." Considering this fact, it's also ironic that she won't sleep with him on the grounds that he's too ugly. The ultimate irony of Donna, however, is revealed at the end of the novel: she's actually been an undercover narcotics agent the entire time. Donna comes to light as a triply ironic character, although by that point Arctor's mind is too far gone to appreciate the irony.
Irony of New Path
New Path is a rehabilitation center, a place where junkies can come to be cured of their addictions. The irony of New Path is immense and depressing, however; the rehab center itself is actually the main supplier and distributor of Substance D, the dangerous drug that has been ruining people's lives. At the novel's end, Arctor ("Bruce") finally realizes this and sends word to Mike, doing something good with his last conscious actions and accidentally redeeming all the mistakes he made along the way.
Irony of Freck's Suicide Attempt
Charles Freck, completely and utterly depressed, attempts to kill himself by overdosing on poisonous drugs. His attempt, however, fails in an impressively ironic manner: instead of dying, he accidentally takes a ton of hallucinogenic drugs and falls into a massive hallucination that promises to last a thousand years.
Irony of Arctor's Condition
Bob Arctor is an undercover narcotics agent, attempting to locate the source of Substance D in order to prevent more people from becoming addicted and ruining their own lives. Ironically, in the process of locating Substance D, Arctor himself becomes addicted to the drug, destroying his mind and life prospects, the very eventuality he was trying to avoid.
