Items pulled apart so you can see what is going on ...

Note the iris is pulled back ... the pupil is detached, pulled back, and scaled up.

The material is a multi-sub ...

Reflection slots with Fresnel Falloff of a city lights image in Sub-materials 1 and 2 ...

Materials 3 and 4 are diffuse ... 2, the iris, is a photoshop image made with smudging black and brown outwards within a circle ... 3 is only black.

You can make it better by casting a specular-only light to enhance reflections, making your own reflection map, working with the iris art more, and making soft reddish in the back of the eye whites.

Additionally, you can consider there variations:

Make the cornea cover the entire eye, and/or;

Make the iris material have its own reflection as well, so it appears to react to light.

Also, the pupil “hole” can be dilated and contracted; more on that in animating the eyes.