Irony is that space between what is expected and what actually happens or exists.
Begin with the landscape: it is desolate yet beautiful. Daru is isolated in the desert, yet it is here that he feels comfortable.
It is ironic that Balducci expects Daru to comply, even though it is clear that Daru (living in exile) is sympathetic to the natives. He has learned Arabic, teaches them and feeds them. But Baladucci believes Daru will be treated as a foreigner if anything occurs: "If there's an uprising, no one is safe, we're all in the same boat," he warns.
The primary irony of Camus' story is Daru's belief that not making a choice is the just thing to do. But not making a choice is a choice. Daru is blinding himself to the realities of the Arab's world. He gives the Arab "dates, bread and sugar," enough to hold out for "two days," as well as "a thousand francs." Despite his gesture, Daru's generosity means very little, for the Arab has little choice but to face the "the administration and the police." They are, Daru says, "expecting you."
The last irony is that both Daru's intended good will and his denial to stay uninvolved has backfired. Upon returning to his classroom, he sees that someone has written on his map, "You handed over our brother. You will pay for this." Like the Arab, there is no way to "win."