It looks like everyone is all Irene'd out from the chapter thread...
Well, just for future references these are links to some crucial Irene info...
She is the Queen of Dragnof
Irene was/is supportive of Dragon-human coexistence
Irene was Queen before the Dragon King Festival
Explains her enchantments
Belserion was saved by Irene's predecessor
Irene came up with the idea for Dragon Slayer magic
Irene begins to dragonise with Erza in her womb
If I am to speculate, Belserion will actually be someone of importance in the civil war. I don't think he'll be anything like Igneel - especially as a warror, but given his position in Dragnof as counsel to the Queen and initiator of DSM, he has to have some role. Maybe not as big as the 5 dragon parents, but he should be known to them. I just hope Belserion isn't Igneel's brother or anything...
The Dragon King Festival probably starts when Acnologia starts dropping bodies. He reaped souls, and created a graveyard. I don't think Irene ever met Acnologia, nor is she concerned with him right now (other than it interfering with Alvarez plans). So I'm going to assume Irene's motives aren't related to a vandetta against the Black Wing; and this could possibly stem from the fact she is his mother
I do wonder if we'll get to see Irene's predecessor and what that person did for Belserion. I hope he/she has red hair (let it be a man pls, Shanks

). This leads to my next speculation - Neinhart. He calls Irene
and Erza 'Lady' as in royalty. We know Irene is now a Queen. So it's not a leap to say Neinhart is from Dragnof as well. Neinhart was able to discern Erza's heritage by her hair, so I'm thinking Irene's lineage have scarlet hair (seems pretty obvious). Therein lies the problem for why Neinhart was brought along to the current timeline, I mean why? He could be one of the last survivors of the kingdom, or Irene needed assistance after giving birth. Or, Neinhart is just from the current era, and knows a lot of Irene's past for some reason. At any rate, Neinhart knows, so I assume there's a reason for it. Even if it's just to dump info to Jellal or someone else.
Irene is apparently irreversibly dragonising. Tbh, she shouldn't be able to detach such a thing because it's a transformation rather than attaching magic to her human body. But she was able to turn Hisui into a mouse, so maybe with her powers she was able to save herself. Other than that, the only thing I can come up with is Irene giving birth to identical twins, and she fixes her personality/soul to one of the newborn <<< this is really cutting it thin though.
I still have no reason for why she wants to kill Erza, other than how Erza says it 'being a bad omen that they meet on the battlefield'.