The principal irony of the story is perhaps that although Daru has, in effect, released the Arab prisoner instead of escorting him to the police as he was supposed to do, he finds the ominous message left for him in the schoolroom after he returns. It implies that a good deed doesn't go very far or have much meaning in this situation. But what, one might ask, could Daru have done? Would any action under the circumstances have accomplished the goal of defusing the tension between the "occupiers" and the "occupied"? It's ironic that a straightforward question such as this has no clear answer.
The message of the story is one in which Camus expresses sympathy for the indigenous North African population. Daru, like Camus himself, is a liberal who recognizes the injustice of the colonial occupation. Yet those of French descent who had themselves been born in Algeria saw the country as their "home" as well. The French title, "L'Hôte," can mean either "guest" or "host," so an underlying irony is that Daru, who serves as the "host" of the prisoner in one sense, is just as much the "guest" of the Arab man because the French are occupying his country.
Yet, even those French who were progressive in thought did not believe, at the time the story takes place, that they could simply clear out and "go back" to France. Eventually they did decolonize, but it was only after years of brutal war. The right-wing French factions then attempted to assassinate DeGaulle for having made the decision to grant independence to Algeria.
One of the ironies of the story is that Daru is happy living like a monk teaching school in a remote district because it removes him from direct involvement in politics and war (although he does have to teach colonialist history and geography to his students) but, as he finds out, he cannot escape such direct involvement. The last thing he wants to do is transport an Arab prisoner to the authorities in Tinguit, yet, ironically, that is exactly what he is called on to do.
Ironically, too, Daru gives the Arab every chance to escape and find freedom—he wants to Arab to escape—and yet the man refuses to take advantage of his opportunities. This forces Daru to do what he doesn't want to do—both because he wants to be left alone and because he rightly fears reprisals from the prisoner's Arab comrades—and deliver the man up to the authorities.
In a final twist of irony, the aloneness Daru has so treasured makes him vulnerable to revenge, for it means there is no one to protect him from the Arabs. He comes back to his schoolroom to find scrawled on his blackboard:
"You handed over our brother. You will pay for this." Daru looked at the sky, the plateau and beyond the invisible lands stretching all the way to the sea. In this vast landscape he had loved so much, he was alone.
All of these are examples of situational irony, which occurs when events work out the opposite from the way a character plans or wishes they would.
The irony in Camus' "The Guest" is that Daru wrestles endlessly with the weight of making the correct moral decision, constantly weighing all the variables, contemplating his place in the world, and resenting the conflict around him altogether. Yet when the time finally comes to make the right choice, and he does so boldly, the result is that it didn't matter at all what choice he made; the outcome was the same, and he may as well have done the opposite of what he actually did.
After reluctantly accepting the Arab prisoner and agreeing to take him to the sheriff—even though, as a schoolmaster, none of this is his responsibility—Daru ruminates heavily on the nature of this conflict in which he is swept up. Daru sets the Arab free, gives him some money, and offers him advice on where to go to escape. The prisoner, however, turns himself over to the sheriff anyway, and when Daru returns to his schoolhouse, he finds a threatening message from the prisoner's comrades.
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."