Since the boy idealizes Mangan's sister, the fence may symbolize the division between reality and the infatuated illusion of the boy in "Araby."
Or if Mangan's sister came out on the doorstep to call her brother in tohis tea we watched her from our shadow peer up and down the street. She was waiting for us [Mangan and the boy], her figure defined by the light from the half-opened door. Her brother always teased her before he obeyed and I stood by the railing looking at her. Her dress swung as she moved her body and the soft rope of her hair tossed from side to side.
From the "sombre" shadows of the houses and the "dark, muddy lanes" of his neighborhood, the boy see Mangan's sister as an image of Mary, almost saintly with a light behind yet, yet seductive in her movements and tossing of her hair. However, he is held at a distance from her by the "railing." This symbolic railing, suggestive of a communion railing in an Irish Catholic church, maintains its motif throughout the story as the boy never has real contact with Mangan's sister. For instance, when he invites her to the bazaar, she cannot come because she is going on a religious retreat.
The boy's other religious imaginings--carrying parcels on his Saturday shopping, he imagines,
I bore my chalice safely through a throng of foes. Her name sprang to my lips at moments in strange prayers and praises...my body was like a harp
However, the relgious "railing" closes these romantic dreams for the boy as the reality he finds at the bazaar is less than exotic and romantic. After he arrives late, he hears only petty gossip and the tingling of coins. Letting his "two pennies fall against the sixpence" in his pocket, the boy's eyes "burned with anguish and anger" at his self-deception in his idealized and religious images of Mangan's sister.
The fence physically separates the narrator and Mangan's sister, but it also symbolizes how untouchable she is emotionally. The boy exalts and worships Mangan's sister so much that she seems almost unreal, or at least unreachable to him.
In a cultural sense, the fence also symbolizes how "cloistered" Mangan's sister is, as well as the other young women in her convent school. She mentions that she cannot go to Araby because of a school event. One gets a sense that the girls of Richmond street do not enjoy quite as much freedom as the young boys do.
The fence symbolizes the fact that the sister is unattainable. The protagonist, realistically, will not be able to woo the sister, who is not interested in him romantically. She is his first real crush and this is evident by his actions, like neglecting everything else in his life to watch her. He finally realizes, though, at the end of the story, the reality of what time he has wasted obsessing over this young woman. This reality is a sobering one for the young protagonist. He realizes that he has much to learn about life.
Well, think about it. The fence symbolizes something different to every reader depending on how they interperet the rest of the text. To me, however, I remind myself of how many girls (and boys alike) put up walls to pretect themselves from being hurt by others. Also, as shown with history, fences have been built by countries, dynasties and homeowners all to keep the unwanted out. But on occasion these fences are built to keep the wanted in. So does Mangan's sister want out or to stay in? For example; The Great Wall of China, The Berlin Wall, and the fence in your neighbors backyard. Put that all together and you have a symbolic fence.