Theosis ("deification," "divinization") is the process of a worshiper becoming free of hamartía ("missing the mark"), being united with God, beginning in this life and later consummated in bodily resurrection. For Orthodox Christians, Théōsis (see 2 Pet. 1:4) is salvation. Théōsis assumes that humans from the beginning are made to share in the Life or Nature of the all-Holy Trinity. Therefore, an infant or an adult worshiper is saved from the state of unholiness (hamartía — which is not to be confused with hamártēma “sin”) for participation in the Life (zōé, not simply bíos) of the Trinity — which is everlasting.
This is not to be confused with the heretical (apothéōsis) - "Deification in God’s Essence", which is imparticipable.
Now they don't explain here why it is imparticipable (Greek philosophy - Platonic thought ala Proclus, et. al.- look it up), because Ireneaus doesn't speak in regards to the Trinity, but calls Jesus and the Holy Spirit God's hands.
Hunt around the site for all the doctrines presented there by the Eastern Orthodox. They are really worth exploring and you can see that as the church ages past the 2nd into the 3rd century the doctrine regarding the Godhead begins to change and the Trinitarian formulation begins.
More than Catholics, the EO have kept closer union to the teachings of the early Christian Church.
Here you can read about the EO opinions on the teachings of Ireneaus.
you need to read more on the development of the Trinity as well as Ireneaus, rather than relying your dubious, unnamed sources. Trinitarian formulation is present in the creeds of the NT and can be followed through the 1st century. still striking out christian fundamentalist