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.