The primary element of irony embraces also the main theme of the story - that is, that life is a sum product of consequences of one's personal choices and that of simple destiny, over which one has no control.
Hardy deals deftly with both factors, leaving the reader to mull over which seems to finally have the upper hand. The author leans to a more fatalist denouement, though, as if to say that once a person sets a certain course of action into motion, it is difficult thereafter to "undo it:"
Judging by Michael Henchard’s end, though, Hardy’s message seems to be that each choice a person makes limits future choices and that a single bad choice can put a person forever at the mercy of blind, uncaring fate. Michael Henchard can be compared to a seaman in a storm who, in a moment of carelessness, loses his grip on his ship’s wheel and is never able to regain control of his course.
- from eNotes.com/mayor-casterbridge/themes
The very fact that Henchard got drunk when trying his best to avoid alcohol, his natural weakness, sets off the "domino effect" of the rest of the story. This is a very clear example of dramatic irony, where the reader (and character) expects one thing to happen but when, in effect, the opposite happens instead.
paradox - A statement which seems to be self-contradictory but which upon closer analysis really isn't.
denouement - The unwinding of events once a crisis point has been reached . (From French, literally meaning "the untying of the knot.")