The irony of the stepmother
Anna isn't actually poor, except that her stepmother robs her of her inheritance. That's why Anna lives as an impoverished person, because of one specific instance of evil. Although many people are familiar with the archetype of the evil stepmother, it's still ironic, because mothers are supposed to represent love, care, and help, but Hester represents the opposite. She represents cruelty, selfishness, and betrayal.
Anna's sexuality
Anna wants to be loved, and she wants a vibrant, healthy life. Not to mention that she begins the novel as a curious virgin. Her natural curiosity about sex is confused by her social status, because women in the lowest class are regarded as prostitutes in the novel. Another way to say this is that in the novel, Anna is not allowed to be sexual. Whereas the men walk away without consequence, Anna ends up pregnant, evicted, injured, disenfranchised, and worst of all, she ends up ashamed for her sexuality.
Ethel
When Ethel recruits Anna, the job seems to good to be true. Why would Ethel want to hire Anna to be a manicurist without any training or background? The answer is revealed to the audience before Anna realizes it (dramatic irony): Ethel is the madame of an underground brothel.
The failed abortion
To abort a pregnancy is ironic, since the purpose of sexuality is partially to create children, but to Anna, the pregnancy represents a horrible fate. The fact that the abortion ends up in catastrophe is doubly ironic, because it means that in any case, Anna is facing a dilemma about her future life (if she survives). This scene shows the conclusion of serious mistreatment and abuse where Anna is constantly punished for sexuality by her society. Men's sexuality goes unchecked the entire novel, but Anna ends up punished for anything and everything she could possibly be punished for.
The hope for the past
The novel ends in a similar way to the way it begins, with Anna dreaming of her Caribbean homelands. This time, she looks back to the past to find hope for a future. She must remember times in her life that were beautiful and passionate and fulfilling, because it gives her hope to believe that life might get better one day. It's highly ironic that a sparky Caribbean woman should be condemned to such a harsh place in England of all places. (England is pretty gloomy, and the constant, gray cloud cover stands in sharp contrast to the Caribbean's hot sun).
