What was the irony of Sally's getting married?
Throughout The House on Mango Street, the reader is aware of Sally's sexual promiscuity. At first Esperanza thinks her friend's worldliness about boys and sex is cool. Later Esperanza realizes that all of it is an attempt to escape (or way of of coping with) the brutality of her father. Sally frequently comes to school with evidence of her father beating her. She says the stereotypical "I fell" to try to cover it, but that is simply not true.
Late in the story Sally marries a man in an attempt to finally get out of her house and away from her abusive father; however, in order to marry such a young girl, the man has to move with Sally to another state where it is legal to marry a girl that young. The irony of her marriage is that Sally's new husband treats her just as poorly (if not more so) than her father treated her. The husband is violent and controlling. He doesn't let Sally leave the house, make phone calls to friends, or even look out of the window. For all of her attempts to escape that kind of abuse, it's ironic that Sally winds up right back where she started (or worse).