A late 2nd/early 3rd century manuscript fragment of Irenaeus’ Adversus Haereses has been found in Egypt; and Athanasius’ theology incorporates several themes of his. When the young Gregory conversed with leading theologians in Alexandria, presumably they knew Irenaeus. In Cappadocia, Gregory of Nyssa also borrowed from him.
In Demonstration 11-12 and Adv. Haer. 4.38, Irenaeus portrays Adam as a child instructed by God who easily fell through inexperience. Gregory incorporates these very distinctive ideas in Oration 38.12, part of a key theological summary repeated verbatim in Or. 45. Moreover, Irenaeus says in Adv. Haer. 3.23.7 that in mercy God gave fallen humans death to halt their sinning and liberate them from it, an idea Gregory repeats in Or. 38.12 ad fin. As an ancient writer, Gregory does not cite his source, though interestingly he presents these Irenaean concepts as his own speculations.