St. Irenaeus is being literary here. The Church Fathers delighted in doing unto the New Testament the same thing that Christians had already done to the Old – namely, using scripture quotes in every possible context to demonstrate how it all related to what they’d heard from the Word Himself.
I should first point out the context, which was making heretics admit they were twisting the Scriptures, making up false gospels, and pulling logically inconsistent beliefs out of their butts and claiming Jesus and the Apostles had taught them “secretly”.
In the third book, Irenaeus turns to a somewhat more positive approach – refuting the Gnostics by telling them what the tradition of the Church actually was. He elaborates on what Jesus taught and what the apostles taught. (He follows up Against Heresies with another huge tome on the Apostolic Teaching, which is apparently just as interesting as Against Heresies.) The quote you mentioned is immediately preceded by a preface summing up the previous two books and announcing this third book’s premise.
"Call to mind then, the things which I have stated in the two preceding books, and, taking these in connection with them, thou shalt have from me a very copious refutation of all the heretics; and faithfully and strenuously shalt thou resist them in defence of the only true and life-giving faith, which the Church has received from the apostles and imparted to her sons. For the Lord of all gave to His apostles the power of the Gospel, through whom also we have known the truth, that is, the doctrine of the Son of God; to whom also did the Lord declare: “He that
heareth you, heareth Me; and he that despiseth you, despiseth Me, and Him that sent Me.”
The things heard from Jesus. The things heard from the apostles. The things taught by the Church. In other words, scripture and tradition, both safeguarded by the Church, the pillar of the Truth.
The title of Chapter I then reads: “The apostles did not commence to preach the Gospel, or to place anything on record, until they were endowed with the gifts and power of the Holy Spirit. They preached one God alone, Maker of heaven and earth.”
So then we read your quote: “We have learned from none others the plan of our salvation, than from those through whom the Gospel has
come down to us, which they did at one time
proclaim in public, and, at a later period, by the will of God,
handed down to us in the Scriptures, to be the ground and pillar of our faith.”
So in other words, the Church (which he knows the Christian reader will remember at the reference to “the pillar of our faith”), has handed down the Scriptures from Jesus and the Apostles.
So yes, the Scriptures are one of the important tools that the Church uses. But Irenaeus is not denying that the Church is the pillar of the faith. Rather, he is binding together the Church and the Scriptures, and questioning why anyone who’s turned away from the Church towards Gnosticism would be messing about with the Scriptures at all – much less, claiming to know the Scriptures better than the Church which handed them down.
I do invite anybody who’s interested to read the Church Fathers. There’s nothing better than early Christian literature to put
all our petty problems into perspective. Also, they lived among unbelievers a lot more than most Christian writers, so they’re actually more helpful for modern problems than you’d expect. Irenaeus is probably not where to start, though, unless you’ve got friends with Da Vinci Code on the brain.
The best way to read the Fathers, though, is out loud. (That’s how people back then read, actually. St. Ambrose was famous for not moving his lips when he read.) Check out
marialectrix.wordpress.com or other podcasts containing material by the Fathers for free public domain audiobooks of their stuff.