Erza was the source of Irene. It happened when she was tortured in the tower of heaven and lost her eye. The give away is Irene's lion staff. Irene clearly parallels Sekhmat, the eygyptian lion goddess who loved causing suffering and was a manifestation of the eye of ra (which was the RIGHT "eye" eygyptian symbol, vs the eye of horus which was the left). She was the protector of Pharoahs (zeref) and led them in warfare. She was a solar deity usually depicted dressed in red (scarlet). She was the harsh aspect of the lunar godess of joy and love, Hathor (erza) and was known as the "mistress of dread"/ "the red lady" (the scarlet despair) and "mighty one of enchantments". One story is that she was transformed FROM Hathor by ra to punish humanity, and as "the right eye of ra" did so until the gods (?zeref ? Ankersham) grew weary of it and tricked her into getting drunk by making a vast amount of beer look like blood, which she drank.
During her childhood torture, erza lost her RIGHT EYE, either through her own actions or that of a protector (eg a fairy of origin or a god). Lots of fan angst has been spent on the fact that she wins because "she's erza", but if she is protected by a god tier character it is actually important that the protection is evident in the story. It is simply too perfect not to be the basis of these two characters. Irene must be erza's right eye, formed during her torture as a child to punish humanity.
Looking into this, a very large amount of fairy tail is eygptian beyond just the obvious talking cats. But what does it mean for the story? Was it just inspiration? Who knows. In the end, the repair of ra's right eye is complicated and once repaired it becomes Wedjet/Nekhbet, a nurturing godess with pendulous white breasts who protects horus (?natsu) from Set and marries..... get this..... Hapy, a water God named "lord of the fishes" and depicted as a blue man. Their child was nefertem, Lord of perfume (or should I say parfum). Obviously ezra isn't going to marry happy and ichiya is not their son, but she may, once she repairs her right eye by merging with Irene, do what sekhmet did and marry the first person she sees (guess who). And I think at the very least we can trust Hiro to make further use of wedjet/nekhbets "pendulous white breasts" in his drawings beyond just the recent calming of natsu and grey.