It certainly hurt them in the early going of 1996. They likely wouldn't have started 2-3 if Irvin hadn't been suspended.
There are a lot of factors there that have nothing to do with Irvin however. I mean if Irvin didn't get in trouble does that mean Lett wouldn't have been suspended later that year? Doubtful. Leon probably still would have been and that would have still hurt the defense in a bad way.
If Irvin had been there those first 5 games would Kevin Smith have magically not been a shell of his former self? Doubtful. That achilles injury destroyed him and Irvin playing, or not playing, had absolutely nothing to do with that fact.
Now lets say he doesn't get hurt against Carolina and we do win that game and go to Greenbay. We would NOT have gone to Greenbay and beaten that Packers team in Greenbay to make it to the Superbowl.
People can hope and wish for that all they want but if we had to go to Greenbay for the NFC Championship game in 1996 the Cowboys defense would have been destroyed. They couldn't stop the run without Lett, so Greenbay would have pounded the ball nicely on them, and Kevin Smith would have been torched by a wide array of WR's.
Now if we could have had a better start to the year and got them in Dallas I'd say we'd have had a chance, despite those factors, simply because Favre sucked so bad in games in Texas Stadium. That's the only reason we'd have had a shot. But the thing is I don't know if we got to the Superbowl that we'd have even been able to win. Glenn would have eaten Smith alive and the Patriots certainly would have run the ball all day long on use without Lett. So that's just way to much speculation as to what they might have done there.