I thought if you don't use Adobe Lightroom to work on the eyes of your portraits, you can always sharpen them using Photoshop. This is the most quick and easy way to accomplish this process in Photoshop.
Step 1 - Duplicate the background layer
It goes without saying in all my Photoshop blog post that the first thing you should do is to duplicate the background layer, thus eliminating the unwanted error and working in a non-destructive way. To duplicate the layer just hit Ctrl + J (PC), Cmd + J (Mac).
Step 2 - Zoom close
Get the Zoom tool (hit Z) and zoom close on one of the eyes. Click on the mouse left button and release when you are happy with the closeup.
Step 3 - Sharpen Tool
Get the Sharpen tool from the toolbox. It looks like a triangle and it is located just under the Blur tool. In the Option Bat select "Protect Details" checkbox and lower the Strength amount to around 20%. Reduce the brush size using the '[' key and paint a few times over the entire iris, including the outside edge of it.
Step 4 - Layer Opacity and Blending Mode
Like everything in Photoshop, too much of everything is bad so we need to blend the Sharpening for a smoother effect. Lower the Layer Opacity as needed. Around 70% worked well for me.
Now change the Layer Blending Mode from Normal to Luminosity so it sharpens the details and not the colors.