The following words written by Reinhold Niebuhr in 1952 sum it up neatly:
Irony consists of apparently fortuitous incongruities in life which are discovered, upon closer examination, to be not merely fortuitous. Incongruity as such is merely comic. It elicits laughter. This element of comedy is never completely eliminated from irony. But irony is something more than comedy. A comic situation is proved to be an ironic one if a hidden relation is discovered in the incongruity. If virtue becomes vice through some hidden defect in the virtue; if strength becomes weakness because of the vanity to which strength may prompt the mighty man or nation; if security is transmuted into insecurity because too much reliance is placed upon it; if wisdom becomes folly because it does not know its own limits – in all such cases the situation is ironic. The ironic situation is distinguished from a pathetic one by the fact that the person involved in it bears some responsibility for it. It is differentiated from tragedy by the fact that the responsibility is related to an unconscious weakness rather than to a conscious resolution. While a pathetic or a tragic situation is not dissolved when a person becomes conscious of his involvement in it, an ironic situation must dissolve, if men or nations are made aware of their complicity in it. Such awareness involves some realization of the hidden vanity or pretension by which comedy is turned into irony. This realization either must lead to an abatement of the pretension, which means contrition; or it leads to a desperate accentuation of the vanities to the point where irony turns into pure evil.