Welcome to the forum.
I am happy to see that you are interested in adding other plants to your iris garden!
Any monoculture planting is boring, but a monoculture of irises not in bloom is not only boring but unattractive (imho; I know that others may not share that view
).
That said, you haven't told us what state/zone you live in, which will factor into what companion plants you can use with your irises.
As others have noted, what companion plants you can use will also be dependent on what your local critters are, which may feast on your plants. Groundhogs, gophers, rabbits, deer will all eat one plant or another. Fencing is the only thing that I have found which works to keep deer out. (From what I have read, it seems to be a mixed bag as to whether or not deer will eat irises. They never did here, but at that time I had far more daylilies than I did irises, and they
loved the daylilies.
)
I am in Zone 9, and given my current critter problems (gophers and rabbits) right now the companion (non shrub) plants I am growing with the irises are very limited... For perennials I have
Iberis for evergreen foliage and white flowers during the winter into the TB bloom season, society garlic (I have both white and lavender flowering cultivars; they provide a very long season of color, starting in April and ending some time in October), a very dark blue flowering
Agapanthus, a CA native
Limonium, and some daylilies. For annuals, I have
Pelargoniums (which generally overwinter here but will usually take some degree of damage), and I alternate between annual vinca and primroses for warm and cool season color. I also have some landscape ("Flower Carpet") roses and a few azaleas planted near the irises, but somehow I just don't consider shrubs to be "companion plants".
