I´d aim-constrain 2 locators to the eyes, one for each eye. that prevents eyecrossing when the distance between eye and locator is very small in case you´d have only one locator for both eyes. additionally you could decrease the distance between the locators a bit to simulate squinting when the target is very close to the face. then group the two locators or controlObjects together or maybe parent them to a targetControl. make this targetControl a child of the head or headJoint. place them controls in front of the eyes or wherever the character should look. beware of the axis that should be constrained. Change the Aim Vector fields so they read 0, 0, 1 in case the iris points along the z-axis.
cheers!
everything starts and ends in the right place at the right time.