As for common language, I think what HM means is that a "realist" is usually someone who denies the existence of things, such as fairies. When people in international relations hold a realist) stance, they aren't saying that they believe in something that their opponents don't (as philosophers would), but that they believe in less, while their opponents hallucinate altruism or believe that treaties are ontologically basic objects.
I think you are wrong to say that utilitarians are generally classified as realists. They are only erratically so classified. When you say "even though both make ethical claims reducible to psychological claims," it sounds like you are defining realism as cognitivism.
"Realism" in the philosophical sense has to be relative to something - Plato's essences, "collective imagination", society, truth are among the subjects that evoke comments that this person is a realist (considers the "something" to be real), and that one isn't.
A philosophical realist w.r.t. fairies is one who believes Fairies are real, while the non-realist says talk of fairies is due to overactive agency detectors or some such thing. It will tend to seem like the opposite of everyday use of the word "realism" -- at least if the subject is one non-philosophers would ever talk about.
I mention this only because I found it a bit difficult to get, and I think I've now "got" it. Correct me if you think I'm wrong.