Well, sorry to start my question with disagreeing with you but actually I think you mean situational irony. Remember, when we think of irony there are three types that we can identify and talk about: verbal, dramatic and situational. Verbal is when something is said that is actually the opposite of what is meant. Dramatic is when we know something that a character doesn't, and lastly situational irony is a sudden reversal of fortunes that occurs.
Clearly, when we consider this excellent short story, it is the last type of irony that Jackson uses to great effect. The situational irony of course begins when we realise the actual fate of Tessie Hutchinson once she has "won" the lottery, but of course for you in your essay you need to identify why this surprise ending is such a shock. You might like to start examining the setting and the characters.
The setting it seems to me deliberately misleads us by predicting a pleasant, happy community day. Note how the very first sentence seems to achieve this:
The morning of June 27th was clear and sunny, with the fresh warmth of a full-summer day; the flowers were blossoming profusely and the grass was richly green.
There is nothing in this setting to indicate the horrendous events that close this story. Likewise, the majority of characters continue to behave quite normally, joking together and not acting as if human sacrifice is just about to occur. Even Tessie Hutchinson, when she arrives late, jokes with the organiser:
Mrs Hutchinson said, grinning, "Wouldn't have me leave m'dishes in the sink, now, would you, Joe?," and soft laughter ran through the crowd as the people stirred back into position after Mrs. Hutchinson's arrival.
To me, it is a combination of both of these aspects of the story--the physical description and the actions of the characters--that help create the pleasant mood that then makes the situational irony we experience in the final page so shocking. Of course, after reading the story once, we can go back and identify the various ways that Jackson foreshadows the grisly end, but it is testament to her great skill that we do not suspect something until the final pages.