Irony is employed throughout The Crucible. Before the play even begins, the girls are found practicing witchcraft in the forest with Tituba. By Mary Warren's own admission we know Abigail "drank a charm to kill Goody Proctor." However, to protect themselves, the girls point their fingers at innocent people who are quickly found guilty- with no evidence against them save the girls' testimony.
We also see irony in the courts as those who are not guilty must lie and say that they are or face execution. Those who do not chose to lie die as guilty witches.
Susanna Walcott works for Doctor Griggs. She is one of the girls in the forest as well as one of the girls "afflicted" by witchcraft.