I’ve been creating some eyelashes and eyebrows for a character and have tried using the Particle > Mirror operator (not to X Mirror toggle under “Keep”), but it doesn’t work no matter how. I’ve tried using it by selecting all the hair on one side, I also tried unselected, tried doing on a cube in a new file (cube > created hair > set amount to 0 particles > particle edit mode > draw on one side > mirror > nothing happens).
If you can manage it Id suggest making a floating plane of faces for the eyelashes that you could rig to the eye lid later, do the eyelashes on one side then duplicate the eyelashes and s x -1 making sure to have the mirror mode set to the middle of the models head. Particles currently is buggy so this l think will prevent you having to try to get it working on the large head. Hope this helps.
If it’s still can be helpful for anyone:
In order to the “Particle > Mirror” function to be applied, your mesh topology must be perfectly symmetrical. Most likely, you applied a decimate modifier to it, so you’d better go to Edit mode and make the mesh symmetrical before making any hair particles