I can bring single iris rhizomes to bloom in 1 gallon pots, and those are shallower than 12". ("Single" because that is all the room there is surface-area wise, in a 1 gallon pot.)
Wrt raised beds, I agree with everything Andrea said. My daylily seedling beds are 12" deep, and those seedlings do great (they need more water than irises do, so I water daily with a drip system). As Andrea said, the dirt WILL pack down over the years, so every time you dig and divide, it's probably not a bad idea to add more dirt/compost in addition to other soil amendments such as fertilizers. And yes, raised beds (as with pots) will dry out faster in warm weather than the ground does.
I would raise two cautions wrt any raised bed anywhere. The first is, to make sure that tree roots can't invade. (Yes, I know you said rock... but just in case.) If they can, they will suck all the moisture/nutrients out of your bed, and you will risk losing your plants. (That is why we have been rebuilding my daylily seedling beds, so they are raised up above the ground and tree roots can't invade.) And yes, even irises can struggle under such root invasion circumstances. (PCIs seem to be able to cope with some degree of tree roots, but the bearded irises will have a challenge just surviving, and you can forget about
I. confusa or
I. japonica.)
The second caution that I would have, is what kind of rock are you dealing with, and will it leach or otherwise affect the soil pH in that area?
(Limestone would make the soil more alkaline.) Your irises won't be able to take up soil nutrients if the pH isn't right.
You should also give some consideration as to what materials you will be using to fill your raised beds with. The past few years I have been potting irises into a 50/50 mix of a commercial planting mix and fine redwood mulch (which breaks down over time into a compost). When we fill the seedling beds (as opposed to just filling a pot), we also throw in vermiculite to help with drainage, about 48 qts per every 30 cubic feet of the 50/50 mix.
Good luck with your raised bed - I'm sure we'll all be interested in how it turns out. (Maybe you will use rock for the walls of the bed? That would seem appropriate.)
