What is the situational irony in "Young Goodman Brown"?
Situational irony describes an occurrence that is the opposite of what we expect. In this kind of ironic situation what actually happens is so contrary to our expectations that it mocks our human intentions and the confidence we have in our ability to plan our futures and take control of our lives. An example might be a haughty rich man who will come begging from us tomorrow, or a woman who is dreading the party she is going to that night but actually meets her future husband there.
In "Young Goodman Brown" therefore there is a whole load of situational irony. Goodman Brown sees the people he suspects least of having an involvement with the devil to be actually witches and involved in meeting with the devil. For example, Goody Cloyse, Brown's "moral advisor" has a conversation with the devil about a witch's brew. Just at the point when finally Brown decides to go no further so he can face his minister and deacon with a clear conscience, his minister and deacon pass in a carriage discussing the meeting they are going to have to worship the devil. And of course, finally, his wife, Faith, is going to be at the meeting too. How ironic!