The best example of dramatic irony in the play comes at the end, when the County Attorney jokes about the quilt that Minnie was sewing. Earlier, he makes a comment that the method of the murder is clear; the victim had a noose knotted around his throat until he died of asphyxiation. However, there is -- to his eyes -- no hint of the motive.
COUNTY ATTORNEY: ...If there was some definite thing. Something to show -- something to make a story about -- a thing that would connect up with this strange way of doing it --
COUNTY ATTORNEY: [Facetiously.] Well, Henry, at least we found out that she was not going to quilt it. She was going to -- what is it you call it, ladies?
MRS. HALE: [Her hand against her pocket.] We call it -- knot it, Mr. Henderson. [(CURTAIN)]
(Glaspell, Trifles, etext.virginia.edu)
The irony is that this is, itself, one of the major clues to Minnie's guilt. She was unhappy in her marriage and wanted to escape, but could not; a sign of her unhappiness is her sewing, which was knotted but slowly became worse and worse. When her husband killed her canary, she killed him with a knotted rope. The Attorney, not caring about the sewing or other "trifles" that he sees as women's issues, laughs about their concern for the quilt and inadvertently receives the solution to the murder: Minnie made the decision to "knot" her work and kill her husband with a knotted rope. Had he been more interested in her home life and the circumstances which led up to the murder, he might have been able to see past his prejudice and discover the truth.