First make a circle filled with the color you want your iris to be, with a diameter greater than you want for your iris. Keep your circle selected.
Then first part of the trick: Menu > Filters > Noise > Add noise > Gaussian, monochromatic. Tweak the amount of noise as it will change the texture a lot (here 10%)
Second part of the trick: Menu > Filters > Blur > Radial blur > Zoom method. Again, tweaking the amount of blur gives different results (here 50)
As you blur, the edges fade. Menu > Select > Modify > Contract, then
inverse to get rid of the faded edges:
Now play with levels / curves / whatever to get closer to the color you want:
Add a black circle at the center for the pupil:
Then burn the edge of the iris and around the pupil:
Almost done, now apply lightning/shading to the iris with dodge and burn:
Here is your textured iris ;)
The only thing that is a bit tricky is to add the right amount of noise at the beginning, you'll have to try :) Here i made a quick one without refining a lot, but you got the idea. Here's a nicer result with exactly the same method:
