Grigson couldn't pick RGIII without Irsay's blessing. Irsay has to sign off on who ever we pick, and all signs point to Irsay wanting Luck. I believe Polian was on ESPN earlier this week saying that Irsay believes Luck is the right fit for multiple reasons and that Luck is definitely headed to Indy. Can't remember the exact quote though.
Irsay has the final say on all major decisions (coaching/GM hires, releasing Manning, what we do with the number 1 pick, etc).