Detached means something not connected to any other structure or building, separated from the rest or isolated. In streets dead-end, at the very end of it, it's common to find some type of detached building, like a house; and such isolation don't tend to bring anything interesting or exciting to people around it, but dullness.
In James Joyce's "Araby," the narrator uses light and dark imagery to set the tone and mood of the story and also to describe the nature of life on the street on which he lived.
