How would you say "some friend" or "some book"?
For instance, if your friend didn't help you after you dropped all your things, or if they betrayed you, you would say "Some friend". You're ironically saying that he/she is a good friend, when he/she, in reality, is not.
2 Answers
You could use an expression like vaya before the phrase. For example, ¡vaya (un) amigo! (Irónico) -> some friend he is! With the right tone it could be used for irony.
That's sort of an idiomatic usage. It's just a known phrase in the English language. Idioms, a lot of times, don't have direct translations and can cause a lot of confusion for people trying to learn another language. Like for instance "it's raining cats and dogs" means it's raining very hard to an English speaker because it has become standardized, but if you translated that directly to another language, they might think you're crazy.
As far as irony, I suppose you can say anything that's obviously not true. It just depends on your tone of voice, facial expressions etc whether or not you're conveying the meaning to someone else. So for that I think you could say "¡Qué buen amigo él es!" or What a good friend he is...