To the OP: here's 2 cents from lurker land.
Regarding Irenaeus, theosis, and LDS use of Irenaeus' writings on the topic, what Irenaeus believed is rooted in the God he worshipped: a God who is uncreated, immaterial, pure spirit, and who created all things out of nothing, including matter itself. This is the same as what Catholics believe today. Irenaeus was a Catholic bishop after all.
“This then is the order of the rule of our faith, and the foundation of the building, and the stability of our conversation: God, the Father, not made, not material, invisible; one God, the creator of all things: this is the first point of our faith. (Demonstration of the Apostolic Preaching: 6)
“The rule of truth which we hold, is, that there is one God Almighty, who made all things by His Word, and fashioned and formed, out of that which had no existence, all things which exist.” (Against Heresies 1.22.1)
“…[The Gnostics] believe not that God, according to His pleasure, in the exercise of His own will and power, formed all things (so that those things which now are should have an existence) out of what did not previously exist...But they do not believe that God (being powerful, and rich in all resources) created matter itself, inasmuch as they know not how much a spiritual and divine essence can accomplish...
“While men, indeed, cannot make anything out of nothing, but only out of matter already existing, yet God is in this point preeminently superior to men, that He Himself called into being the substance of His creation, when previously it had no existence.” (Against Heresies 2.10.2-4)
But the things established are distinct from Him who has established them, and what have been made from Him who has made them. For He is Himself uncreated, both without beginning and end, and lacking nothing. He is Himself sufficient for Himself; and still further, He grants to all others this very thing, existence; but the things which have been made by Him have received a beginning. But whatever things had a beginning, and are liable to dissolution, and are subject to and stand in need of Him who made them... (Against Heresies 3.8.3)
“Truly, then, the Scripture declared, which says, ‘First of all believe that there is one God, who has established all things, and completed them, and having caused that from what had no being, all things should come into existence: He who contains all things, and is Himself contained by no one.’” (Against Heresies 4.20.2)
I'm not sure why LDS folks like to quote Catholic bishops like Irenaeus in support of LDS claims. Why not quote an ancient Christian who believed what the LDS Church teaches about theosis - that the goal is to become like the LDS Heavenly Father (who is a divinized, embodied human male who created the world out of pre-existing matter, similar to what Platonic elements in Greek philosophy teach about the origin of matter).