Irons is going to have a lot to say about John - this is right. In my own research I found dozens of suggestive passages in this gospel, and it is the boldest in the claims it attributes to Jesus, and the absolute quietest on the Kingdom of God. One thing Irons rightly highlights is John's emphasis on Jesus coming "in the flesh". Why say that Jesus came in the flesh? That sounds like a strange thing to say if you did not have to defend against faulty thinking about the Christ, one that came from God not in the flesh. So it is readily agreed and understood to be an early or prototype anti-docetic affirmation, a heresy that for some reason Irons neither mentions nor explains. A reason for this perhaps is his keeness to show that the pre-existence of Christ is simply self-evident in such a passage as this. It is a good point that he needs to make in defense of a Trinitarian view, but here it needed better situating, which would have nuanced his quick conclusion somewhat.
Irons leaves John briefly to visit Paul, anticipating the argument that John's writings are late first century, allowing for christological development. He heads unswervingly to Philippians 2, the great, and early christological poem (p12). However, like with some translations (the NIV being among the worst in this instance), Irons does a little paraphrasing. He states:
"Jesus' decision not to regard equality with God as something to be used for his advantage".
Here the lexical decisions are all made for us and all ambiguity removed without any discussion whatsoever. That is a pity. The Greek word used here by Paul/the poem is rare and difficult to trace within the canon, but outside it is used. Main usage seems to be (and I have checked this in other ancient literature) to pillage, take what is not already owned, like in a raid. So by assuming that this equality with God is something that Christ already has, Irons is steering the translation of Harpagmos away here from a common definition of the word. This assumption ignores other probable (and incompatible) meanings, justified primarily by the exact place he wants to take his readers: pre-existent coequality, which is a separate argument to pre-existence alone. This is therefore a little misleading given the subtitle he provides the reader (simply referring to pre-existence in Paul). A more likely translation is therefore "grasped after", and a more neutral translation that leads both options open (like the NET translation) is simply "grasped": He did not consider equality with God something to be grasped, full-stop. The Greek does not allow for anything more.
Now Irons continues by conflating that Jesus:
"humbled himself by becoming a man".
This is a conflation of the Greek. Irons strikes me again as slipping in his conclusion too early into his argumentation. If you want to look more into this you can check out the "raw" Greek here. There is not space here to discuss Philippians 2 more, but I recently did a post on the conclusion to the poem here.