Mary Maloney calls Patrick "darling" many times. He never responds with a similar term of endearment. In fact, he is dismissive and condescending. He breaks the news to her with no compassion whatsoever.
When Patrick gives Mary the news, she thinks that she can convince herself that it has not occurred.
Perhaps, if she acted as though she had not heard him, she would find out that none of it had ever happened.
Later, after killing him, Mary employs the same strategy. If she acts like she is innocent, it will appear as though she is innocent.
And now, she told herself as she hurried back home, she was returning to her husband and he was waiting for his supper. She had to cook it well and make it taste as good as possible, because the poor man was tired; and if she found anything unusual or terrible when she got home, then it would be a shock and she would have to react with grief and horror. Of course, she was not expecting to find anything unusual at home. She was just going home with the vegetables on Thursday evening to cook dinner for husband.
When the police arrive, she makes it a point to tell them that she's put the lamb in the oven. She says, "it's there now." She is telling them where the murder weapon is. Of course, this is part of the irony that is to come.
Jack Noonan refers to the killer as "he" and "him." He, and perhaps the other detectives, assumes that the killer is male. Mary's acting has worked.
As the detectives eat the murder weapon, one says "she wants us to finish it." Not one of them considers the lamb to be a weapon. None of them suspects Mary. Obviously, she wants them to "finish" the murder weapon, making it impossible for them to attribute the murder to Mary.