There is a white line that is about 2cm long.it is underneath my eye ball, pretty deep. mid way of my eye socket. it causes irritation once in a while?

See Eye MD: A doctor must look at your white line to identify and treat this properly. It may be a foreign body that got stuck and needs to be removed. The conjunctiva may have folded over itself and rubbed itself causing irritation. It may be a string of white discharge from dry eye or allergy. You can try to rinse it out. If not successful, have eye MD examine and possibly remove it.

Eyelid?: If it is of the eyelid it may be a benign condition called xanthelasma. See doctor.