I'm guessing we just roll with 52 players this week. Seems pretty likely that they cut Smith to make room for Irvin. Anybody they sign would just be cut a week later, and they can't really call up any PS guys because they would be guaranteed 3 weeks on the roster (or 3 game checks, not sure which). Either way, it's not worth it. We are fine at 52 players for a week.
As far as the players involved, Smith has looked pretty useless in two games. Even with very high effort he is a non-factor. If they did keep him, his only value would be as a 1-tech backup, but McDonald can do that just as well and he's a far better pass rusher. McDonald had a solid game against SF (no official stats but he was in on some pressures) and a monster game against Jacksonville.