St. Irenaeus had certain historical information available to him, we don't know how much.
Based on the data he had before him he seems to have thought that Christ's ministry lasted twenty years.
I never claimed that St. Irenaeus was inerrant. I never even made the lesser claim that he was infallible.
Ireneaus seems to be quite certain of his statements. At the very least, it opens the door to the possibility that Jesus' ministry lasted longer than three years and that Jesus lived longer than we are led to believe.