Epsom salt is an inorganic compound containing magnesium and sulfate.
Some people think that Epsom salt can be used to nourish flowers, but this has not been scientifically studied. When you pour it on the soil near the roots of your irises, it will not provide any nutrients to the flowers themselves. If you want to enrich your irises with magnesium or sulfur, you should use other products like fertilizer or compost instead of Epsom salts.
It is documented in the Texas A&M AgriLife Extension, Department of Horticulture and Landscape Architecture, "that Epsom salts are often mistakenly used to treat chlorotic (yellow) leaves. Chlorosis is caused by lack of chlorophyll or iron, which can be corrected by adding iron or any fertilizer with an adequate balance of nitrogen (N), phosphorus (P), and potassium (K). Epsom salts contain neither of these nutrients."