From what I can gather, the average adult human iris diameter is about 12mm, so superimposing that onto your sensor size (about 22mm by 15mm) gives you an idea of what you'll get through a 1:1 lens like the one you mention. Other considerations will include:

• getting a subject to hold very still (not just their head, but their eyes) and/or being able to work fast, if you want to have artistic control over the outcome and be able to focus precisely

• getting light where you want it, given the small distance between the lens and the subject, and enough of it so that you can work with the small apertures needed for reasonable depth of field, and in such a way that it isn't harmful to the subject

• not causing your subject to blink due to TTL pre-flashes, meaning you'll probably want to set the flash power manually. Doing so will also help with the next point:

• you'll need to be able to keep the camera and subject from moving AT ALL for the brief time between acquiring desired focus (by whatever means) and when the flash fires (at macro scales, even otherwise imperceptible movement can change the point of focus from the one that you wanted).

The reflection on the left side hints that this photo was taken with a flash fired through a grid (you could use something like this DIY version) from the side. This gives a very focused directional lighting that will bring out the texture. Using flash will allow you to adjust ambient lighting to allow for the amount of pupil dilation you want.

A small LED light stuck on the flash can provide contrasty, but not overly bright, focus-assist lighting. I've actually built LEDs into black straw grids like the one linked to, in which case it also serves as somewhat of a modeling light, so that you can see where the flash is going to hit.

That's what I can think of from a general macro-shooter perspective; someone with more eyeball experience may have more to add.