Irony or sarcasm? I'm not always clear on the distinction. I have a feeling that Answers to this Question could run dozens of pages long.
Affirmations can be used to mean the opposite, for example.
In der Tat (indeed). Sicher doch (of course, certainly).
One German stand-up comic became famous for
Riiichtisch (= richtig; yeah right).
Das haben Sie fein gemacht: If your boss tells you that, you should be worried. (Situational context and tone of voice matter.)
He is not exceptionally smart has a counterpart in
Er ist nicht besonders clever or sometimes
Er ist nicht besonders helle (the latter being even more dismissive of the person's intelligence).
If you say
Ich würde Herrn X nicht unbedingt widersprechen (I wouldn't necessarily disagree with Mr. X) and then proceed to demolish Mr. X's argument, context reveals your true intent.
You ask about superlatives and negation.
Großschriftsteller (major writer) is one timely example.
Suboptimal means lousy, awful. If I think of more examples I'll add them later.
EDIT: There was something about your question that bugged me and I can think I can put my finger on it now. I'm not entirely sure* that the construction
He is not exceptionally X is inherently ironic. To me, it takes a bit more (tone, context) than the bare naked construction to convey the speaker's intent. What do others think?
*And I do mean "not entirely sure"; I'm not being sarcastic :)