This quotation is found in "Antony and Cleopatra," Act 5, Scene 2, IRAS says, "Finish, good lady; the bright day is done, And we are for the dark."
In this final scene, Cleopatra is trying to commit suicide, but she is stopped. Dolabella feels sorry for Cleopatra, and he informs her that Octavius is planning on parading her through the streets of the city. He wants to show her off as the prize for the war.
Octavius comes in to speak to her and tells her she better not try and kill herself because if she does, he will harm her children. This is where Iras tells Cleopatra to "finish," meaning get the killing over with. She is visited by a clown who brings her a basket of figs and beneath the figs are asps. The snakes bite her servants and they bite her. All three women die.