Dramatic irony is where the audience knows something that characters in the text don't. In the case of Trifles, as the play draws to a close, we know something that Mr. Henderson doesn't know. He thinks that Mrs. Hale's and Mrs. Peters's talk about quilts is just yet another example of a female trifle, something trivial and unimportant. But on the contrary, the quilt is very symbolic in a number of ways, and if Mr. Henderson and the other men in the play had understood this, then perhaps they'd have got to the bottom of Mr. Wright's killing.
Just after having discovered the dead bird with its neck broken, Mrs. Peters and Mrs. Hale are interrupted by the sheriff and the county attorney. The women quickly hide the box with the bird inside.
COUNTY ATTORNEY. (as one turning from serious things to little pleasantries) Well ladies, have you decided whether she was going to quilt it or knot it?
MRS. PETERS. We think she was going to—knot it.
Here, the audience, along with Mrs. Hale and Mrs. Peters, is now aware not only of the way in which Mr. Wright was killed but of the similar way in which the pet bird was killed. The women hide this piece of evidence from the men, who remain oblivious to the symbolic nature of the unfinished quilt and the significance of "knotting" it.
At the end of the play, when Mrs. Hale repeats what Mrs. Peters had said earlier—that Minnie Wright was going to finish off the quilt by knotting it—she isn't literally talking about quilts. Rather, she is referring to the method in which Minnie killed her husband. Mrs. Wright was going to finish off the quilt by knotting it in much the same way that she finished off her husband by putting a rope around his neck.
However, as we've seen, Mr. Henderson doesn't understand Mrs. Hale's subtle meaning. In his typical male chauvinistic way, all he hears is what he regards as a silly woman going on about silly female activities like quilting.