Mrs. hale. it’s log cabin pattern. pretty, isn’t it? i wonder if she was goin’ to quilt or just knot it? (footsteps have been heard coming down the stairs. the sheriff enters, followed by hale and the county attorney.) sheriff. they wonder if she was going to quilt or just knot it. (the men laugh, the women look abashed.) what is ironic about this excerpt?

I believe that what is ironic about this excerpt is that the men believe the women are incapable of discovering anything of substance when, in fact, they are the ones who discover evidence.
The women are the ones who are able to see such subtle clues as her stitching, which is something that men don't usually pay any attention to. However, her stitching gave women a reason to believe that she was disturbed by something, and ultimately come to the bottom of the whole crime that happened, which is quite ironic given that the "detectives" couldn't do it.