To fully understand the irony of this statement, it's first important to understand the context of Mr. Wright's death. Mr. Hale, a neighbor of the Wrights, stopped by to see John Wright one morning but instead found Minnie Wright quietly pleating her apron. When Mr. Hale pointedly asked to see John, Minnie responded that this was impossible because her husband was dead. When pressed further, Minnie indicated that John had "died of a rope round his neck."
Mrs. Peters and Mrs. Hale are left alone and quietly investigate the crime scene; through various details that only the women notice, they realize that Minnie almost certainly killed her husband. They are also cognizant of the mistreatment and isolation Minnie has endured in a male-dominated world. There can be no true "jury of one's peers" in a court system that forbids women from serving on juries; this story was written decades before all fifty states passed laws to include women on juries.
As the full truth becomes clear to Mrs. Peters and Mrs. Hale, the county attorney tries to make small talk with the women. His demeanor implies that they can't possibly add any "serious" substance to the investigation, as he flippantly asks whether Minnie had been planning to finish the quilt by quilting it or by knotting it. Mrs. Peters hurriedly answers that Minnie was planning to "knot it," which demonstrates verbal irony because they realize that Minnie had designed a far more deadly knot—the one used to kill her husband. The truth is lost on the attorney, who merely comments that the choice of a knot is "interesting, I'm sure."