The double irony in Gene's comment of Chapter 8 of "A Separate Peace" is this:
1. Gene suspects Finny of deception when, in reality, Finny has no deception in him. For instance, when Gene fails a test after having been called out the night before to a meeting of the Secret Suicide Society, he suspects Finny of wanting him to fail. However, on another night before an exam, Finny again asks Gene to come to the tree; but, when Gene declines this time, Finny accepts his excuse, indicating that his intentions have never been ulterior.
2. The reality is that Gene is the one who the deceptive one. For, when Gene is on the branch with Finny he deliberately shakes it out of jealousy for Finny, causing him to plummet to the ground, breaking his leg. In truth, it is Gene who is jealous of Finny:
He had never been jealous of me for a second. Now I knew that there never was and never could have been any rivalry between us. I was not of the same quality as he....and then my knees bent and I jounced the limb...and jumped into the river, every trace of my fear...forgotten. (Ch.4)
The greatest irony of all is that Gene cannot relieve his anxiety over his relationship with Phineas. As Ronald Weber writes in Studies in Short Fiction:
It is Phineas's innocence that Gene cannot endure. As long as he can believe Phineas shares his enmity, he can find relief, but with this assurance gone, he stands condemned before himself, and must strike out against his tormentor.