By Reputation, August should win since he's supposed to be the strongest Spriggan. But if we compare him and Irene and their respective magic, it would leave some plot hole if August beats Irene.
So what do we know about August's copying magic? It cannot copy caster type magics which makes sense since he wouldn't be able to summon a celestial spirit without a key no matter how strong he is. But other than that, any magic should be fine.
But if that is the case, then he should also be able to copy DS magic. But then, that would make him a DS as well. And the Spriggans was assembled in the first place to face Acnologia, so shouldn't August have DS magic already? If he had, then why didn't Acno attacked him when he killed God Serena.
And the thing is, one should not be able to beat a dragon no matter how strong he is unless he has DS magic. So that's where the plot hole will enter. If he will be able to beat Irene just because he's stronger, then the whole premise of DS will be useless.
So August indeed may be stronger than Irene power wise, but based on their magic types, he may not be able to beat Irene. But that doesn't necessarily mean Irene would be able to beat August since August should be close to unli magic supply.
The thing with DS magic though, it's not just about learning the element, it's changing your body's Constitution. So August may be able to copy poison magic from Cobra, but does that mean he already learned DS magic? It was never specified in the manga so we would never know.
But by virtue of August's "can-copy-all-magic" magic, should he copy Irene's DS magic, then that's GGWP lol..