I recently read the last FT chapter and something came in my mind.
We don't know too much what Irene is capable of. So I thought what if Irene can predict the future just like Carla. Expect she predict it much more furhter. And she saw everything happen in this current arc.
Maybe she did everything in her life on purpose to stop Zeref. We don't know what connection she has with Erza but she discarded it, so she could leave Ishgar. And I think she gave Erza to the Tower of Heaven in the first place to get rid of their tie. But as we know Erza survived the Tower of Heaven and became a powerful mage herself. Because Irene didn't expected Rob to sacrifice himself young Erza, Irene is surprised to see her alive and she want Erza to be asssssinated because of this.
I think her ultimate goal is to sgain Fairy Heart for herself and combining it with her waste magic power to gain some even more powerful. And with it she wants to eliminate Zeref and Aconologia. Of course these actions will require her own life as well but she's willing to sacrifice everything she has to put an end of those two individual.
Please tell me your thoughts about it. And please if you think did something wrong in this blog, feel free to correct it.