Irony can broadly be defined as something that is the opposite of what you expected occuring. So, taking that as a base definition, there are numerous examples of irony that can be found in act two. The first is in the character of Mary Warren. In act one, we saw her as a terrified, submissive, mouse of a girl who could barely speak for herself, especially in the presence of John, her employer. Well, in act two, she comes home late and boldly stands up to John and Elizabeth, defying their commands for her to remain away from the courts. Her reaction is surprising and a bit ironic.
Then, let's take the arrests that happen at the end of the act. Elizabeth, one of the most righteous and upstanding members of the community, a respected farmer's wife with an outstanding reputation, is arrested for witchcraft. Not only is Elizabeth arrested, but the pure and nearly perfect Rebecca Nurse is arrested for being a witch also. This is bizarre--Rebecca's reputation is so solid and amazing that even Reverend Hale, who comes from Andover, has heard of how amazingly charitable she is. It is ironic that these upstanding women are arrested, while the corrupt and conniving girls accusing them go untouched and are revered as angels of God.
Also, consider how Elizabeth was arrested. Mary was sewing a doll as a present for Elizabeth, and it is that "present" in the end that is evidence for Elizabeth's arrest. Another irony: John's good friends, Cheever and Herrick, are involved in taking Elizabeth away. They claim that their hands are bound. But in the end, it is by friends that Elizabeth is dragged away from her house and family. These examples, and more, are some of the bits of irony that can be found in act two of the play. I hope that those thoughts helped; good luck!