Sorry for not getting back sooner, been a tad busy.

You can build up quite complex shading networks with the layered shader. The eye I made utilises 4 ramps that make use of the blending modes, one ramp controls the viens, one for the overall eye colour, one for lines in the iris and one for an extra underlying noise within the irsis. I used the blending modes to dictate how they show through one another. I just experimented till I got I good understanding of how the structure works.

I should really go into more detail but I am excedingly tired, and it's my birthday, yey.

I may try and do a quick walkthrough of making one for my website when I get the time.