Why is this comment ironic: "They all watch, as Abigail, out of her infinite charity, reaches out and draws the sobbing Mary to her"?
Irony is when the character does or says something different than what the audience would expect of that character.
In this case, the depiction of Abigail is what is ironic. Abigail draws Mary to her as if she were a mother nurturing her child. Abigail is anything but a nurturing person in this story. In addition, Abigail is clearly not a charitable person. She holds a grudge on John Proctor for not continuing the affair with Abigail. She punishes him and Proctor's wife for this. The phrase "infinite charity" also has a Biblical undertone. Abigial, in thestory, is not one who is fervently religious although she pretends to be a devout believer. The audience knows that Abigial, just as she holds grudges, also does things that the church sees as evil: dances naked in the woods and lies to her elders and the church.
The adjective repentant in regard to Mary is also somewhat ironic in that we know that Mary only repents because of her fear of Abigail. She is not repenting to the Lord as a good Christian confessing to witchcraft to be saved but rather she repents as a young girl afraid her friend will be so mad at her that her friend will get her condemnes to death for witchcraft.
Abigail's "charity" for the town includes accusing innocent women of witchcraft, a crime for which many of them hang. She has pretended to be righteous and holy, despite the fact that she has sent numbers of people to ignominious deaths. Despite the fact that the court's job is to suss out the truth and separate it from any lies, the court has, instead, believed the lies and punished those who speak the truth (as Mary Warren briefly tried to do). Danforth tells Proctor that the state believes "the voice of Heaven is speaking through the children." Abigail is not viewed as the vengeful and violent murderer that she is; instead, people seem to think of her as some kind of saint: Elizabeth Proctor describes in Act Two how the crowds part for Abigail when she walks, as the Red Sea parted for Moses. Now, she hugs Mary Warren, a girl who tried to tell the court the truth about her, a girl who Abigail hersel, persecuted in Act One. It looks like a warm and winning scene: the holy Abigail accepts her friend, Mary, back into their righteous cause. Instead, it shows a murderess, welcoming her co-conspirator back to their evil plan.