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Possibly , but I still think a real iris is slightly convex, plus I'm no good at material nodes, and I'm not sure whether refraction would cure the highlight/shadows problem.

that's incorrect.  look up diagrams of the eye, the iris inset from the rest of eye.  and while the rim might not catch the light, if you notice, almost all art including photos, shows  the irs having shadows and being darker beneath highlights and lighter opposing the light  (concave shading) when showing eyes that have depth.  personally, i've observed this every time i've looked in mirrors with lighting from above (as opposed to very ambient lighting).  it's a subtle effect, and easily overwhelmed by ambient or frontal lighting, but it definitely helps determine the appearance of "deep" vs. "flat" or "doll" eyes.

http://www.intrepid.com.au/wp-content/uploads/the-human-eye.jpg
http://www.nyrealestatelawblog.com/j0406700.jpg
http://topouest.com/wp-content/uploads/2009/01/b1eye01.jpg
http://www.meb.uni-bonn.de/cancer.gov/Media/CDR0000543553.jpg
http://www.terrapinphoto.com/jmdavis/eye%20NS8K.jpg
http://acclaimimages.com/photos/aa.jpg
http://www.eyedesignbook.com/ch3/fig3-57bBG.jpg