Of course there is much irony in The Crucible. One of my favorite examples is when Danforth mentions that the court is a place where all concealment is melted down. This completely relates to the title because a crucible is a container that allows for great ability to melt (at high temperatures) strong metals.
So, essentially Danforth is saying that he cannot be lied to in his court. He will seek out through great trial and questioning all truth.
He says to Proctor:
"We burn a hot fire in here. It melts down all concealment."
The irony of this statement is simple. While Danforth warns Proctor, and likely his friends Francis and Glies, that they better not lie under any terms, he is in the process of and continuing to believe lies as truth and judge under the circumstances of accepting lies.