Only because he held to the doctrine of the early church, also. Otherwise, we wouldn't agree with him since he isn't "infallible", only The Word is infallible.
FYI:
Irenaeus and Tertullian had to contend with the Gnostics who were the very first to suggest and teach that they possessed an Apostolic oral Tradition that was independent from Scripture.
These early fathers rejected such a notion and appealed to Scripture alone for the proclamation and defense of doctrine. Church historian, Ellen Flessman-Van Leer affirms this fact:
For Tertullian Scripture is the only means for refuting or validating a doctrine as regards its content...For Irenaeus, the church doctrine is certainly never purely traditional; on the contrary, the thought that there could be some truth, transmitted exclusively viva voce (orally), is a Gnostic line of thought...If Irenaeus wants to prove the truth of a doctrine materially, he turns to scripture, because therein the teaching of the apostles is objectively accessible.
Proof from tradition and scripture serve one and the same end: to identify the teaching of the church as the original apostolic teaching.
The first establishes that the teaching of the church is this apostolic teaching, and the second, what this apostolic teaching is (Ellen Flessman-van Leer, Tradition and Scripture in the Early Church (Van Gorcum, 1953, pp. 184, 133, 144).