(2) You set up a false argument by wondering whether St. Irenaeus is qualified to judge the NT or if he can ever be wrong.
The fact is, the NT had not even been definitively collated by the time St. Irenaeus died - the Church had not yet rendered judgment on the Scriptural status of the various texts of the NT. St. Irenaeus may not have had access to all the data we have access to.
St. Irenaeus opined in error, but (1) he did not repudiate any Christian doctrine in erring and (2) he did not have access to all the chronological particulars - he was merely human. Nitpicking one of the inconsistencies in an enormous work full of such sound doctrine is inequitable.
(3) This is why the Church places the Scriptures on a higher level than the writings of the saints - because the of the disparity between the respective authors.