Definition of Dramatic Irony
A major source of dramatic irony in the play is Big Daddy's cancer, which is kept a secret from Big Daddy and Big Mama until the end of the second act and the third act, respectively.
In Act 1, Big Mama and Big Daddy experience significant relief at this news, which is a lie told to them by Doctor Baugh to keep them hopeful; this false hope makes the other characters very uncomfortable and tense:
BIG MAMA: He's just one hundred per cent. Passed the examination with flying colors. Now that we know there's nothing wrong with Big Daddy but a spastic colon, I can tell you something. I was worried sick, half out of my mind, for fear that Big Daddy might have a thing like--
[Margaret cuts through this speech, jumping up and exclaiming shrilly:]
MARGARET: Brick, honey, aren't you going to give Big Daddy his birthday present?
Margaret's interruption characterizes the dramatic irony that pervades most of the play; she won't even let Big Mama say the word "cancer," as if saying it out loud would make the lie too much to bear. This instance of dramatic irony creates a discomfort in the reader that darkens the general mood of the play.