As usual, people see sack numbers and make their assumptions from there.
Irvin has been towards the tops in the league in pressures per rush. He consistently makes QBs move off of their spot, which really helps the secondary out. I also saw two bull rushes where Irvin moved his guy back into Cam's lap and I believe one of those was Mebane's sack. And I only watched part of the second half.
Add this to the roughly half a dozen obvious holding calls he should have drawn, and he's playing well and improving. He is setting the edge against the run, and when the defense shifts over to the run side of the play, he's pretty much unblockable with a TE. I'd like to see him clean up his game, but that can be said of a lot of guys right now on our team.