I use this story every year when teaching my Grade Twelve students, and I tend to read it out to them out loud and judge their responses to what they hear. Every year it is exactly the same: the students are incredibly shocked and surprised by the ending, that suddenly transforms what seems to be a happy village celebration of some sort into something much darker and more menacing and sinister. Based on the success each year with my students, I would say that the greatest type of irony that is proved effective is the situational irony, that Jackson crafts into her story so carefully to make the brutal ending a real shock. One way you might choose to analyse this excellent use of situational irony is by comparing the first sentence of the story with the last:
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.
"It isn't fair, it isn't right," Mrs. Hutchinson screamed, and then they were upon her.
Hopefully you will identify the massive disparity that is created by this beginning and ending. The initial sentence deliberately misleads us by creating a happy tone, completely obscuring the terror and brutality of the ending that Tess faces. Jackson, therefore, whilst including various elements of foreshadowing, manages to successfully lull us as readers into a false sense of security, making her ending all the more poignant and shocking, as the normal and seemingly human villagers are almost compared to a pack of wild, blood-thirsty animals.