Someone would need to be waived, or sent to IR, to make room for Irvin. Irvin doesn't officially go on waivers until Monday (the Raiders didn't actually make the transaction in time on Saturday), so we probably won't know if a team with higher claiming priority than the Pats has snagged him until later Tuesday. If he's not claimed at a $3.756M cap hit, the Pats might try to sign him for something less on Wednesday. Meanwhile, Simon hist the cap for only $400k.
It's possible that Ferentz is waived back to the PS Tuesday or Wednesday, depending on the status of Schwenke and Mason, so that would be a temporary solution. But then, another spot would need to be made for Dawson by Nov. 13th, and possibly yet another for Burkhead by Dec. 4th or 5th (if he's designated for return and begins practicing the week of Nov. 13th).
Davis effectively missed a full year of football with his bulging disk (beginning with the 2017 Combine), so not being an impact player yet isn't surprising.
Simon had an immediate positive impact with five tackles and a sack in his first game with the team (MIA, wk-4). He also set the edge well. So, it's possible that Irvin could, too (though not certain).
OTOH, it's a question of value. Will Irvin be certain to make a larger enough impact than Simon in the Pats scheme to justify the added expense beyond $400k?