Gosh, off the top of my head here at work, I would say to gouge-out my bosses eyes for reference and.... whoops.... I've said too much....

LOL hehe...

Seriously tho,

I'm assuming your interested in that cool "football" shape that cats and reptiles iris's have right?

You have 2 options that I know of:

1. Use a plain-old texture map of a reptile's iris. (probably your best bet)

2. In the ramp's 2d placement node of a "regular" eye ramp, adjust the..... wait... I just tried it here at work, and after trying EVERYTHING on the 2D placement node, it looks like it cannot be done? weird!!!

Sorry....

What I ended up doing was selecting the "regular" eye blin shader, and SHIFT selecting my eye geometry, and in the hypershade, choose convert to file texture.

Then I opened it up in Photoshop, and tweaked it by hand....

Sorry man....

That's the best I can do from here...