Irony is recognized with a knowing look, an allusion of being in the know, or with one’s spoken references to another level of awareness usually as an attempt at humor.  We have rueful irony or sardonic irony but what one may mistake as irony may merely be earnest stupidity.  Irony is ambiguous, it can be hard to read, it goes over the listener’s head or it is ignored.  “You are a genius” said with a sneer might be insulting or it might be a complement withholding the full intention of the meaning of “genius” because we all know no one really is; never the less the speaker expresses his admiration.