Camus is interested in the existential choices of his principal characters. "Guest," translated from French to English, can mean "guest" or "host." In this way, each character is both a "guest" and a "host" of the others, just as each character in Camus' most famous novel is a "Stranger." The Arabs are guests in the French-controlled country of Algeria. The Colonial French are guests of the native Arabs. Daru and Balducci are both unwilling hosts of the Arab (neither want him).
The principal irony comes at the end. Daru has left the Arab at the crossroads: one way leads to freedom (the nomads); the other way leads to death (the prison). Daru refuses to deliver the Arab to either place; he tries to make a choice by not making a choice, which Camus says is, by default, choosing death. So, ironically to Daru and the reader, the Arab chooses death. Is it because he was honoring his host, Daru? Is it because he was afraid of a nomadic life of freedom?
Camus says that most people are afraid of freedom; therefore, most people--at the crossroads--choose death. They refuse to acknowledge the absurdity of the universe and give up their freedom of choice to external forces (the French gov't; the gendarme; a host, etc...) Philosophical, absurdist irony.