Toiletries and Medical Supplies:
When I was there the tab water was conterminated and we only were told in the pub at night. The only supermarket on the island closes at 5 or 6 so we had no chance to buy drinking water. Just be warned that there's only one shop in case you need one!
Photo Equipment: There are a few tourist shops in Kilronan where you can get things like this - but it's definitely cheaper to bring everything you need for the time you'll be there! After all this is an expensive place and variety is limited!
Miscellaneous: Bring cash! There is a bank on the island - but it only opens twice a week and doesn't have an ATM machine. So get the cash you need (some B&Bs might not accept credit cards!) before going to the ferry!