score:13
Irenaeus doesn't say and he doesn't give any hint.
Unfortunately, that question couldn't be answered by several other experts in the field:
It is unclear to whom Irenaeus is here referring.
— M. C. Steenberg: "Irenaeus on Creation: The Cosmic Christ and the Saga of Redemption", Vigiliae Christianae, Supplements, Brill: Leiden, Boston, 2008, p185.
Though the chain is not as compressed as in IV 20.2, he does it in III 23.3. Here, spliced between two references to Genesis, and followed by a reference to the Lord’s words in Matthew, are the following words of an unnamed, non-Scriptural authority: ‘As a certain person among the ancients has observed: “God did indeed transfer the curse to the earth, that it might not remain in man”.’
— Charles E. Hill: "‘The Writing which Says ...’ The Shepherd of Hermas in the Writings of Irenaeus", in: Markus Vinzent (Ed): "Papers presented at the Sixteenth International Conference on Patristic Studies held in Oxford 2011", Studia Patristica Vol. LXV, Volume 13: The First Two Centuries — Apocrypha Tertullian and Rhetoric From Tertullian to Tyconius, Peeters: Leuven, Paris, Walpole, 2013. p134