I follow the practice of our local grower. She has decomposed granite (water drains straight thru - compost is tilled in yearly) in a hotter zone (90-100 in summer---80's in spring) than mine.

After planting them, thoroughly soak! No more water for 30 days. This forces a deep root system and lessens the chance of rot. Bearded iris are drought tolerant and do not like constant surface moisture.

I imagine that adobe clay is the sort that sets rock hard in summer? Improving it as you have will help. I assume that, by "horse apples" you mean after they have been digested by the horse, and not before?
I and my bearded irises suffer here from wet clay and I amend with sand, rotted cow manure and gypsum. I know gypsum doesn't work with all clays. You would have to look into that. It improves the drainage considerably.
I also raise my beds as much as possible, and they are now close to 12" above the surrounding area.
what is decomposed glass? Can it cut if you have your hands in the soil (weeding etc.)? Anything gritty added to the soil and tilled in will help with drainage.
I agree with iris-gal about soaking the soil then leaving them alone.
Cheers, Jan in Australia

i am hot, dry and have clay soil, you need to water them in the west. personally, i don't think overwatering is an issue when its hot, my temps have hit 117, avg summer day is over 100 nonstop for months... no way could my iris go 30 days without water, ESPECIALLY IF NEW. you need to water them frequently after planting for the first 2 weeks, no different then in other new plant, once you see they are rooted and begin to grow new leaves, i then water my iris for 45 minutes, 3 times a week until temps drop into 70s in late fall. iris love clay, it holds moisture and nutrients and they will excel. they are drought plants, but you need nice big fat rhizomes to ride out the western heat, the water you supply makes the rhizome large, storing its food and water. for the record i have several 1000 iris, they are super easy to grow, but if you have high heat and wind you need to water them in well to establish them.

If I followed Jains recommendation my irises would all rot. The difference? I don't know, but I suspect it is the fact that my garden is dead flat and Jains is on the side of a hill so she has far, far better drainage.
I would indeed water them MORE often during summer, than I do, but we are on fairly severe water restrictions by then.