Dramatic irony occurs when the audience knows what characters in a play do not.
What the audience of Trifles learns, along with wives exploring Mrs. Wright's kitchen, is the reason why Mrs. Wright murdered her husband. They know it was an impulsive act, the culmination of years of suppressed rage bursting out, because of the disarray in which she left her kitchen. They realize, too, that it was a response to her husband killing her pet bird. They find the canary's cage door was violently ripped opened, and they discover the bird carefully wrapped in a silk handkerchief. The women realize that Mrs. Wright hanged her husband in retribution for him wringing the neck of her beloved bird.
Dramatic irony therefore occurs at the end of the play, such as when the County Attorney says:
If there was some definite thing. Something to show—something to make a story...
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