Others have already disagreed with me on this point, but I dont think Irvin is easily replaceable.
When our defense was at its best last season, we were in the 3-3-5 which effectively became our base. The nickel was a starter.
The two who played nickel are now gone (Diggs/Thompson) and the other DBs on the roster did not get many snaps.
Irvin is the only safety/nickel we have (or had). IMO, he is uniquely suited for this role with his physicality, toughness, quick feet and fluid hips. We have other safeties committed but they are traditional safeties. And although we have one CB committed and two more highly likely (now possibly even 3 more) they are all man-cover, boundary corners. None of them are built like Irvin, with his strength and willingness to pound people. Fields might come closest but he looks like a corner all the way to me (at least for now).
On the other hand, one of the reasons we were able to get away with the 3-3-5 so much was because of the outstanding play of Malcom Brown. So, there is a chance we might go with more 4-3 in 2015 to better suit the DTs on campus. if so, that might relieve the pressure here somewhat. But, it's still the Big 12 and a nickel will be needed.
Candidates -- Jason Hall, John Bonney, DeShon Elliott, Jermaine Roberts? (Edwin Freeman is too big now to even consider)
IMO, Irvin fit like a glove. No one else does.