Irvin doesn't have to stuff the run. He doesn't even have to occupy a blocker much. In Pete's scheme, he has outside contain: RBs can't bounce stuff outside of him. If the RB goes inside, we have Wright there to stuff the run, it's what he's there for. Clemons has made his chops in the run game by shedding blocks in space....it's hard to get hands on a guy in space if your're a TE or big OLman pulling. Clemons manages to "ole!" his guy and get into run plays, while keeping outside contain and chasing down plays from the backside.
Irvin is already adept at chasing down runs from the backside. If he can shed some of those blocks, Atlanta will probably not run to his side as much, and just chuck it. He doesn't have the repetoire of moves that Clemons does, but he's much much faster. If Irvin can "mature" his game a bit, we'll be fine there. We'll miss Clem, but it's not a disaster necessarily.