The main character, Michael, meets Hannah, an older German woman, when he stumbles into her apartment building, sick, and she takes him in and nurses him back to health. This, along with the fact that he falls in love with her and they carry on at least a physical affair for quite some time is ironic given the later revelation that she was an SS concentration camp guard during World War II, responsible for the deaths of hundreds. It is ironic that someone who was trained not to show pity or remorse does in fact show pity and helps Michael out of simple kindness.
It is also ironic that he loves her, even later after he finds out about her past. He corresponds with her, and helps her in whatever way he can. So an SS camp guard, and murderer is truly lovable is another irony: that such a person could not only give but receive love, could not only show compassion but could accept it.