In what respect is the ending of the story ironic? What kind of irony is this? What does it contribute to the meaning of the story?

There is irony at the end of the story for two reasons.
 The Arab, who is given a chance at freedom, chooses to travel alone to jail
instead.

When Daru returns home, having provided the Arab
with the means to escape, he is met with a threatening message on the chalkboard that
tells him, "You have handed over our brother.  You will pay for this."  While Daru has
defied the law in refusing to turn the Arab over to the law, and then given him the
wherewithal to escape, which the Arab ignores, Daru is going to be
punished.

The second example is situational irony: the
difference between what we expect to happen and what actually
happens.

This makes the entire story one long example of
irony.  Daru treats the Arab like a guest and will be punished for doing so.  Perhaps
the meaning it contributes is "no good deed goes unpunished" (Clare Boothe Luce).  This
is a rather cynical statement, but in this story, it seems that Daru, a man so happy
with his life, is undone by the kindness he shows a stranger.