Yes, I always figure I need 6"-8" depth for irises -- mine never seem to send roots any further down. And I have very silty soil that does not hold water for any length of time. In fact, in the summer I now put mulch around my irises -- a big no-no for irises, but when I put it down I try to keep it 2" away from the actual rhizomes, and then when it spreads out there is hardly anything touching them. But that helps keep them green through our hot temperatures in the summer, and then I remove the mulch in the fall.
It sounds like you got a great deal on those irises and they grow and bloom where nothing else would. Congrats!
