Aug 15 2014
A co-worker just walked into my room and asked this question. Normally things ending in -is are made plural by changing the -is to -es, as with axes being the plural of axis. (And pronounced /’ees/.) I thought /IrEEs/ sounded too weird, so I tried to think of English cognates to iris for hints before I answered. Remembering iridology, I guessed “irides” (/IridEEs/). He laughed and said he had already looked it up and just wanted to see if I knew (or in this case, could guess.) Irides is correct.
On a side note, things that end in -us most often go plural by replacing the -us with -i and -os with -oi. Thus vir-es is incorrect, as is hippopotam-i.