Very good, HD. Even you are beginning to realize the "systematic view" of the scripture is something man-made. The scriptures had to be intentionally "harmonized" to show "internal consistency."
Irenaeus is credited for many things, mostly his fight against the Gnostic competitors. He is also credited with the re-incorporation of the Old Testament after a period where it was mostly ignored or minimized, because it didn't "fit" the Christian beliefs and because the Christians were not positively disposed towards the Jews. But he realized that without a heavily Christianized OT interpretation the New Testament is without a power base.
He also is credited for calling Eve the advocata, which when retro-translated into Greek reads Paraclete (the Holy Spirit)! Of course, the earliest surviving copy of Irenaeus' work exists only in a Latin translation form Greek, written a hundred years after Irenaeus, and possibly doctrinally "adjusted."
Irenaeus is also credited for "revealing" that Jesus was at least 50 years old when he was crucified (Ver. haer., II, 22.), so his views on whether man can save himself or not are really not so scandalous relatively speaking, considering what else he is credited for.
Iraeneus, as most early church fathers, came out of a pagan culture into the light. For over three hundred years the church fathers struggled with putting together a systematic view as revealed in the Old and New Testament. It isn't a "man-made" view but a view of people trying to understand the very nature of God as He has revealed Himself to us. Iraeneus didn't "Christianized" the Old Testament. Rather he tried to understand the nature of God throught the inspired word of God in the Old Testament and what had just been revealed in the New. You could say the Jews failed to update their scriptures.
As far as what Irenaeus wrote, many of the early fathers came from pagan cultures or had personal opinions on matters. Some of them had some pretty wacky ideas. But these writings are not considered "inspired" as scripture is inspired. They are filled with errors and misinterpretation of scripture. Many of these people were evangalists-not theologians. So one has to look upon their writings from that context.