One of the major ironies comes about while Assef is beating the living daylights out of Amir. For so long Amir has been haunted and pained by the guilt he felt from watching Assef rape Hassan and not doing anything about it that he is surprised at the rush of comfort and relief he feels even as Assef is nearly killing him.
Another facet of this particular irony is the laughter that wells up and rushes out of Amir as he is being beaten. Assef of course expects him to cringe and cry and perhaps even try to escape but Amir just laughs harder and harder as the pain increases.
Another irony is the fact that Sohrab does end up putting out one of Assef's eyes fulfilling the taunt that his father sent Assef's way when they were children and Hassan threatened him with the slingshot.
Another smaller irony is the fact that Sohrab is forced to dance to music as the Taliban have banned any music and dancing for the entire country.