Here is a .gif of the dilation, the finished eye, and the material breakdown:
http://s1.postimg.org/v5iekcdnj/eyedilation.gif
http://s28.postimg.org/56wzjh1a5/eye1.jpg
http://s24.postimg.org/6vb0m0o51/eye_dilation_material.jpg
I’m using a 512 free Iris texture and a 512 normal map to make the reflections/highlights appear to bulge out (might be easier to just bulge it out with the model and get rid of the normal map all together 
Might change that later). Everything else is procedurally generated, so it works fine. The only drawback is you can’t dilate the Pupil too large or too narrow because it will start distorting the image, and clamping doesn’t seem to help. Parallax also has the tendency to screw it up. It looks brilliant with subsurface scattering too, and not at all like glossy plastic!