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"Eisenherz" is a compound noun, thus you can already rule out wikt:en:eisen#German and wikt:en:herzen#German as they're not nouns. "Eisenherz" is indeed "iron" and "heart" and means someone with a "hardened heart" in a good way, because that someone is brave or valiant. Hence "Prinz Eisenherz" is the German translation of "Prince Valiant".
To have a little fun with language, let's look at the other forms, too. "eisen" is either an obsolete adjective meaning "made of iron", or a verb meaning "to freeze". It might be on the way to become obsolete, too, because various derived forms of wikt:en:frieren#German are preferred as far as I'm aware. wikt:en:herz#German, on the other hand, is the imperative form of wikt:en:herzen#German ("to hug"). Compounding those two would be something like "made of iron-do hug!" or "to freeze-do hug!". It makes as much sense as in English, which is none whatsoever.