The title, "The Guest," is ironic. The Arab is a prisoner, however Daru refuses to treat him as one, opening his home and caring for him as if he were a guest.
It is ironic that in this land of desolation, where the sheep have died by the thousands, men have died without anyone knowing, where the rocks break because it is so dry, and the people must rely on relief from another country, that Daru feels like a lord in his small home, though it is humble, little more than a room, and he is alone. He loves the land, he has what he needs to survive minimally, and does not begrudge the land its inhospitable treatment of those who live in the region.
It is ironic that someone familiar with war would be so reticent (hesitant) to carry a weapon, even if it means saving his own life.
There is a certain irony that the Arab has the means to escape and he still journeys to the jail. One would expect him to run away and take the gift Daru offers at a second chance, but the Arab continues on, to deliver himself to the authorities.
I don't think verbal irony is the same as sarcasm. Irony refers to the difference between what we expect to happen and what really happens. (Though some sources will say it also refers to the difference between what is said and what is meant.) If a fireman's house burns down, it is ironic. If a thief is robbed, it is ironic. It can be expressed verbally when what is said is not what is meant, but it may be presented simply as a statement of fact. "Physician, heal thyself" is a famous line that expresses an irony, but it is not necessary sarcastic. It points out an inconsistency which is often what irony is.
Sarcasm is defined as "a sharp, bitter, cutting expression." It can be most obvious based upon the tone used by the speaker. "Do you think you'll have a good day at school?" "Yeah, right!" What is said and what is meant in the response to the question is not ironic. It is sarcastic. The tone is biting; there is a negative component in the response.
If someone said something sarcastic to me, I would never respond with, "Don't be ironic." I would say, "Don't be sarcastic." The original Greek suggested that irony meant "one who dissembles" or lies. However, if we see irony in the thief who is robbed, there is no lie present. If the original use of the word referred to the act of lying, I think it has altered over time and that today we use the term "sarcasm."
This is my opinion. There are people who will say they are the same, and others who are certain they are not.