For stills use a photograph of an eye....

if your serious about eyes......might want to get the book

http://www.renderosity.com/news.php?viewStory=13579

the pros use:
basically it is a SSS shader for the inner eye with a color map ....and then its covered in a transparant sphere with a kick butt reflection map on it.....(its actually 2 separate peices of geo....just like in real life)