Well, that is pretty incorrect. You don't know that they thought Irving was a first-round talent. In fact, it was Bill Parcells who thought Nate Irving was a first-round talent, despite the fact that he basically lost his leg in an automobile accident. The team obviously didn't have him rated that high since he was a third-round selection.
And why would the roster space be limited? We have a bunch of areas we can upgrade on this roster. ILB being one of the primary ones. Irving and Brooking aren't the kind of guys you want being the only dudes on your roster going into training camp at MIKE.
This is assuming something is done with DJ and Joe as well. We are not deep at LB. We have Trevathan, Johnson, Miller, Irving and Woodyard outside those two X-Factors above, which in my eyes are likely to be cap casualties (and I like both those players). Outside Miller, it really isn't that impressive. Another young player or veteran needs to be added into the mix to make the unit a whole lot better. If one of those guys goes down, we are SOL.