that would connect up with this strange way of doing it—

[The women's eyes meet for an instant.]

As the audience knows, there is something that pulls everything together: the dead bird.

Dramatic irony occurs when the words and actions of the characters in literature have a different meaning to the audience or reader than the characters in the work.  The audience or reader has been given additional information or knowledge that the characters do not have. 

A crime has occurred.  Minnie Wright is charged with the murder of her husband John.  It is unclear why Minnie might have committed this crime.  The county attorney; Mr. and Mrs. Hale, the neighbors; and the Sheriff and his wife Mrs. Peters come to look for clues and find things that might make Minnie Wright’s stay in the jail less harsh.

The men in the one-act play Trifles by Susan Glaspell believe that women are incapable of the logic and reasoning necessary to solve a murder mystery.  One of the men even states that women are interested only in “trifles.” The audience knows as the drama progresses that the women become more attuned to the clues which provide the motive for the crime. The men are clueless.

The men do not search the crime scene nor adequately look into the obvious items the women find.  In fact, when the women mention the quilt, the men make fun of the discussion that the women have about 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.

If the men paid more attention, they could have seen that something happened which caused Minnie Wright to ruin her perfect stitches in her quilt.

When the men discover that the women have found an empty bird cage, they have no inkling of the importance of it to the murder.  The County Attorney  sees the cage and asks about the bird.  He mentions was there a cat in the house.  The women tell him that the Wrights did not have a cat.  That is the extent of his evaluation of the cage.  What does he overlook that the audience already knows about from the women’s discussion?

• The cage door has been ripped from his hinges in obvious anger
• The bird came from a man who sold canaries the year before
• The bird would have been company for Minnie since she had no children
• There was no cat
• The body of the bird was discovered with its neck wrung
• The box and the wrapping indicated that Minnie had intended on burying the bird

The County Attorney indicates that Minnie was a poor housekeeper.  Mrs. Hale begins to look around and discovers that Minnie has been baking bread.  She left a few dishes dirty probably because of the death of the bird and her grief.

The County Attorney further implies that Mrs. Hale was not a good neighbor because she has not visited Mrs. Wright in over a year. His judgmental attitude makes Mrs. Hale angry; and she later points out to Mrs. Peters that John Wright was a hard man who would have been hard to live with for any person.  Mrs. Hale discovers that the table has been wiped to a certain point and then stops with the other half messy.  Something stopped Minnie from finishing her work.

When the County Attorney points out that juries are usually easy on women unless there is something specific that they can tie to the woman with regard to the murder, the women exchange looks because they know that Minnie killed her husband because of years of abuse with the final straw the killing of her bird.

The dramatic irony in Susan Glaspell's play Trifles occurs when the dialogue between Mrs. Hale and Mrs. Peters prompts the reader to make connections as to what went on at the Wright house, right when the ladies are making the same connections. The irony is that, when the male characters come on the scene, they feel that the ladies are talking about mere trifles. However, both the reader and the ladies know the truth: Minnie Wright killed her abusive husband, John and the evidence is actually all over the house!

The burst open jars of compote, the erratic stitching, and the unkempt house, are all signs of Minnie Wright's traumatic state of mind prior to the murder. All these things, to the men, mean something different. They merely mean that Minnie is just a very bad housekeeper; but the women know better, and so does the reader

Then, with the add-on comments of Mrs. Hale, we are able to put the picture together: Minnie Wright was once a happy woman, a singer even, who later became estranged, and then abused, by her husband John. 

Finally, when the ladies find the empty bird cage, and then find the dead bird with its neck wrung, the last connection is made: it is the bird's death what finally hits Minnie so hard that she basically explodes and kills John in a similar manner. 

This is why the play is successful: the entire time, the men have been gone from the scene  and yet the reader is "in" on the little secrets that the ladies are finding. This causes a literary interaction between the reader's own schema and that of the characters. We get mad, along with the ladies, when we hear the men's condescending ways like, for example:

SHERIFF- Well, can you beat the women! Held for murder and worryin' about her preserves. 
COUNTY ATTORNEY -I guess before we're through she may have something more serious than preserves to worry about. 
HALE- Well, women are used to worrying over trifles. [The two women move a little closer together.]
COUNTY ATTORNEY- [With the gallantry of a young politician.]
And yet, for all their worries, what would we do without the ladies?

If the ladies were worried with trifles, they would have never found the clues that the men would surely have wanted to find, if it had not been for their petulance. Therefore, the finding of the evidence, the profiling that is suggested by Mrs. Hale, and the final connection to the dead canary is something only known by the reader, and by the ladies. This is what makes the play dynamic, and what makes the dramatic irony allow it to succeed.