As a member of the original group of accusing girls, Mary Warren has enjoyed a great deal of power. In fact, in Act Two, she explains to John and Elizabeth Proctor how her testimony against Sarah Good -- that the old woman cursed her when Mary turned her away without food when she came begging one day -- was counted by the court as evidence that Good is actually a witch. All Mary had to do was make the accusation and it was believed: that is powerful indeed.
Further, Mary Warren ought to have power as a person coming to the court to tell the truth: the very thing the court is meant to uncover and uphold. Figuring out what is true and what is false is part of the very job the court has. However, it is ironic that, now that Mary has come to tell the court the truth, she is powerless to impact the way the girls are seen. As Danforth tells Proctor after Proctor explains why he has brought Mary to the court, "the entire contention of the state in these trials is that the voice of Heaven is speaking through the children." Mary isn't powerful when she tells the truth; she is only powerful when she lies. Considering the fact that this is all taking place in a court of law -- a place where people are supposed to tell the truth, the whole truth, and nothing but the truth -- it is quite ironic as well.
Mary Warren's interrogation, in act three (scene three) of Arthur Miller's The Crucible, forces Mary to confront Abigail Williams. Mary, a servant of the Proctor's, fears Abigail. Since Abigail controls all of the girls, and is single-handedly responsible for the arrest of many of the residents, Mary fears what may happen to her if she turns against Abigail (in the open).
The irony of Mary's statement, "I have no power," lies in the fact that her interrogation leads to the arrest of John Proctor. She, in "reality," does have power. Through her claim of innocence, Mary is attacked by Abigail. Abigail claims that Mary has sent out her spirit on her. John, furious at Abigail's deceit, calls her a "whore." After this outburst, John must admit how he knows that she is a whore.
Also, when earlier threatened by John (after Mary states she does not have to listen to him any longer), Mary states that she is an official of the court. If she was an official of the court, it would be assumed that she would have some power. To go into court later and claim that she possessed no power would be ironic.