A quite specific instance of irony immediately precedes the boy's epiphany in the story.
At the bazaar, the boy overhears a distinctly banal, playful conversation being carried on between a young woman and two young men. The banter they carry on is far from romantic.
...he overhears a conversation between an English shop-girl and two young men. Their talk is nothing but idle gossip.
The irony in this moment stems from the idea that the boy has come to the bazaar driven by a romantic impulse. Romance is not a part of the real dealings between the men and women he encounters. Real dealings between the sexes, in the example of the overheard conversation, are rude and playful.
The boy realizes that he has been following a false idea as he leaves the booth where he overheard the conversation. The lights of the hall go out at one end, bringing the boy back into the shadows once again (a place he occupies for much of the story, like one of the figures from Plato's "Allegory of the Cave").
Ironically, the boy comes to the realization that he had been blind as the lights go out.
'Araby' uses the "quest" motif in an ironic fashion. In a quest, the hero goes in search of something which will ultimately prove enlightening and significant, overcoming many obstacles on the way. Even though the whole story is ironic, the end heightens the ironic features in the story. For instance, the protagonist of the story who sees himself as a romantic hero, is made to appear gauche, extremely young and foolish in contrast to the self - assured young people at the fair. His pretentions of being a romantic hero are shown up for what they are, in the presence of young adults who are able to interact without self-consciousness with the opposite sex. Then again, his hope of finding something that would forge a connection between himself and the girl he had a crush on, are dashed when he realizes that he had no idea what to purchase; and that the things on sale at the fair were ordinary mundane things. Somehow his grand quest had become tawdry and ordinary; and he himself, had shrunk in stature, from a romantic hero to an inept, self-conscious boy.