I created a procedual iris-shader in blender and continue to refine it.

1. Node-Group is a spherical gradient texture to darken the outer part of the iris or lighten the inner part.
(Color-Mix-Node on Multiply)
2. Node-Group is a spherical gradient texture for the overall color of the iris. Multple Colors can be combined. (Color-Mix-Node on Multiply)
3. Node-Group is a radial gradiant texture combined with a noise texture for the iris-fibers.
(Color-Mix-Node on Multiply)
4. Node-Group is a spherical gradient-texture for the pupil.
(Color-Mix-Node on Multiply)
5. Node-Group is a voronoi-texture bended with a second texture-mapping node on "normal" mapping.
(Color-Mix Node on a 80% Dodge, and Inputs reversed)

All Texture-Mapping Nodes except the second node of 5. have a location of (-,5, .-,5, -,5) and "Generated" texture Coordinates