MG soils tend to hold too much water when used by themselves, combine that with a container with no drainage and your asking for trouble. I would take the plants and all the soil out and drill some holes, then mix the soil with wood chips or shredded bark and some real wood charcoal for added drainage. Do not use charcoal briquettes, some types have added chemical in them and the natural charcoal is various sizes which will also increase drainage.

That your barrel hoops are loose is a good sign the wood is not rotting. When hoops have done that I have done the basic wrap thick wire or braided cable and tighten with pliers. I'm sure there are better and better looking solutions.The thick "copper" wire sold by Bonsai nurserys could work."Copper" because it's mostly aluminum,coated with copper, wire.Not very strong but your not asking it to do much more than hold the staves together.

26" diameter is wonderful! I would be tempted to put 6 TB's in there if they were ones I wanted to save. If buying I would do 3-4. Yes, dwarf iris do take less space. Go with your heart. You will then be thrilled every time it blooms. Were your Grandmother's all the same? If different colors are they in separate locations for you to get a piece of each? How many colors? etc. etc. If the soil is diggable, you can dig up pieces like those marked with a circle. Now. The 2 X's are dead mother rhizomes. This clump has a bad growing pattern. Some do, some don't. In need of serious clean-up. One piece is enuf to start a clump. That piece will be broken off its mother and should be left overnight for the cut area to dry (callus). A raw cut put into soil immediately is vulnerable to rot. The rhizome will look like a small potato. Leaves may not be visable yet in your zone? This link has much better pictures and explanations. Here is a link that might be useful: http://www.schreinersgardens.com/how-to-grow-care-for-bearded-iris

A half whiskey barrel holds about 25 gallons of potting mix. I have "successfully" planted one seed potato per five gallons of mix in containers ranging from 20 to 50 gallons. I say successfully because I've never produced a large crop in a container. The best yield I got was growing red norlands in a container that was wider than it was deep. I got close to 15 pounds of potatoes per seed potato. Potatoes don't really need more than about 12 inches of soil, and do better if they are more widely spaced. One caveat about whiskey barrels is that you have to use very fast draining soil and make sure your barrels has a lot of drainage holes. Soggy soil is a killer.