You will need a camera that can focus in on the eye. What I mean is don't try and do this from ten feet away from the subject. You need enough pixels across the eye to make good measurements.
Try image subtraction. Start by taking a picture of the eye looking right ino the camera. Subtract other images from that one, and calculate the center of gravity value. You can find those VIs in the vision toolkit, under particle analysis.
You don't really need to track the iris to detect where the eye is moving. Think outside the box. It also might be useful to use a camera that is IR sensitive, or has had the filter removed. This way, even dark eyes will appear light in color to the vision system.
