One is a divergence between sense and reference. In the first of these the description connotes more than, and in the second it connotes less than, what is required by the thing to which it refers. The other kind of semantic contrast occurs whenever the contrast between the stated and the unstated meanings is so complete that one is a negation of the other. To sum up. Both ironies of scale and ironies of opposition instantiate a semantic contrast between the stated and the unstated meanings of ironic utterances.
Semantic contrast is thus a necessary condition of irony.
We now come to the third necessary condition, and we can best approach it by considering the difference between hyperbole and meiosis on the one hand, and bombast and euphemism on the other. The purpose of the latter is to conceal or disguise the truth. When a man who empties dustbins is called a sanitary engineer, this is designed to conceal or disguise the fact that his job is menial, smelly, and poorly-paid. When a dictator is called Our Great Leader this is meant to conceal his mediocrity and his fear. Both euphemism and bombast may have unstated meanings for at least some people, but their intention is really to minimise and eventually to abolish any unstated meanings.