It's fine as a name. My niece had swimming lessons with a girl named Irys this summer. My sister said that's how her name is spelled. She'd be around 8. A popular regular substitute teacher where I work has a granddaughter named
Iris. She's about 5 now. I think people will think of the flower or part of the eye before they think of rainbows. I usually do, even though I'm familiar with basic Greek mythology.