This would be a great fight to see, since it would be the apex human vs low-mid tier dragon (or sub in August for Zeref to avoid curse problem). I'm going to add the option of the tie
@Seven777 if that's okay with you...
I voted for Zeref. He's an immortal. Irene has great power but it's not at the point where I'd say she has more than Zeref.
Zeref on the other hand has dragon-level durability in my eyes. He tanked IgNatsu's most powerful attack. GMG AF Natsu KO'd Motherglare. Now there's a fair bit of slayer advantage there, but there is a fair bit of difference between IgNatsu and AF Natsu imo. Keep in mind Zeref is still able to continue against IgNatsu's best attack.
So on feats, Zeref wins. Irene would need August' help to sway my pick here (and even then I'm still likely to go with Zeref due to immortality).