Leon Sim (sometime commenter here) has written a cracker of an essay on Irenaeus's understanding of the Old Testament. Of course that understanding is explicitly christocentric and Trinitarian.
Here are a couple of great quotes from the essay:
Not only does Irenaeus see Christ and the Trinitarian God to be the object of revelation, but he also sees Christ to be the subject or agent of God’s revelation. For Irenaeus, it is not merely incidental, but crucial, that it is the Word who spoke to the patriarchs and prophets, and “preach[ed] both Himself and the Father alike,” (Against Heresies 4.6.6.)...
...In one sense, Irenaeus reads the Old Testament Christologically and Trinitarianly because He sees that the Father does everything through the Son by the Holy Spirit. In another sense, he also does so because he holds creation, salvation and revelation together, that is, it is the Father’s purpose in creation to save and bring men to Himself through the revelation of His Word by the Spirit. Hence if there is to be salvation for humanity in the Old Testament, it must be through the revelation of the Spirit-anointed Son...
...According to Irenaeus, therefore, anyone who follows in the non-Christological interpretation of the Old Testament – assuming that the Father, Son and Holy Spirit were unknown or unrecognisable through the Old Testament itself – have followed unbelieving Jews in departing from the true God and any knowledge of Him: "Therefore have the Jews departed from God in not receiving His Word, but imagining that they could know the Father [apart] by Himself, without the Word, that is, without the Son; they being ignorant of that God who spake in human shape to Abraham, and again to Moses." (Against Heresies 4.7.4.)
Download an easier to read format here.
.