Irenaeous just does not seem to be the kind of guy who cares about the kind of evidence that a historian would respect. Consider his argument for there being exactly four gospels from Against Heresies:
The Gospels could not possibly be either more or less in number than they are. Since there are four zones of the world in which we live, and four principal winds, while the Church is spread over all the earth, and the pillar and foundation of the Church is the gospel, and the Spirit of life, it fittingly has four pillars, everywhere breathing out incorruption and revivifying men. From this it is clear that the Word, the artificer of all things, being manifested to men gave us the gospel, fourfold in form but held together by one Spirit. As David said, when asking for his coming, 'O sitter upon the cherubim, show yourself '. For the cherubim have four faces, and their faces are images of the activity of the Son of God. For the first living creature, it says, was like a lion, signifying his active and princely and royal character; the second was like an ox, showing his sacrificial and priestly order; the third had the face of a man, indicating very clearly his coming in human guise; and the fourth was like a flying eagle, making plain the giving of the Spirit who broods over the Church. Now the Gospels, in which Christ is enthroned, are like these.I just cannot help but think that a guy who reasons to the number of gospels from the number of faces on the cherubim is unlikely to have any better reason for believing in the authorship of the gospels.