I was beginning to think she was some sort of mutation based off Erza's lost right eye, I would not have been surprised if some experiment was done. But then Irene said "its been far too long".I hope, for the love of all thats sacred that Irene isn't Erza from the future or some nonesense like that. Pleeeeeaaaassseeeee.... Cuz he already dabbled into the whole time travel thing and tbh I never liked that kind of thing. I wish Irene is Erza's mom or, the very least, her twin. But if its something like she being Erza's bad half and they being like 2 parts of a demon created by Zeref or some other nonesense like that... Please don't hiro!
If im not mistaken on the second last page with a full body shot i think she has a scar running across her stomach, that may be connected if it really is a scar.
Edit: but I do think it has something to do with when erza was imprisoned.