Move the newly created iris image to your eye image. Scale the new layer down if needed to go with the the rest of the eye. Make sure the middle of the iris layers are in the same spot. Change the top iris layer to difference layer mode, temporarily, while you move the layer into the right position. (the image below is in normal layer mode)

Change the latest iris layer mode to lighten and the opacity (20%).

Add layer masks and erase some parts of the iris layer by paiting in the layer mask, use a biggish soft brush.

Create a copy of the latest iris layer, add the Spatter filter to get some uneveness to the lines and get those little shapes that can be found in many eyes.

Create a couple of copies of these last layers (the ones created from the iris image). I have a total of four of these iris layers. Add layer masks to the layers that don’t have layers masks and erase some parts of the iris. Change the layer modes and opacity settings to these layers. I have used lighten and hard light and opacity setting between 20 and 50%. What you can do, is also to rotate some of the iris layers. To get a bigger variance in the iris.

The colurs are quite saturated so I use the Hue/Saturation to take the saturation down and also change the colour slightly.

There usually is some coloured spots in eyes. So create a new layer and paint in some spots. I have used a greenish colour and changed the layer mode too overlay and opacity to 60%. Make a few layers of spots with different colours, layer modes and opacity settings. I have also added a dark ring round the iris, most people have this dark ring around the iris.

Next is the pupil. The pupil is always round but not always totally even. Make a circle and fill it with black, then paint with a hard small brush, little bumps in the circle.

You might want to add a reflection. What I have done here is I have painted a small curved “square” to sort of follow the shape of the eye. I have painted it light blue, used a couple of colours and then changed the layer mode to screen and lowered the opacity just slightly (98%).

This is the final result once again.

What I do is, to make the eye in a separate image and quite big. When I want to incorporate the eye into my facial texture I merge all iris and pupil layers into one layer and the eyewhite/vein layers into one, the reflection is one, I might want to move it or remove it altogether. These three layers are then moved into my facial texture and scaled down to the correct size. I can then add adjustment layers to the different eye layers if needed, to change colour and saturation. The iris had a really sharp edge so I blurred the two outmost pixels of the iris, to get softer edge.
I hope that this have been of help to some of you, please let me know if anything is unclear.

If you like let me know how it goes!