What might give you an irritated look? (9)
I believe the answer is:
eyestrain
But I can't explain it at all!
(Other definitions for eyestrain that I've seen before include "Possible effect of too much reading" , "Stress on mince pies (in the East End)?" , "puts pressure on pupils" , "Discomfort of sightseers" , "pupils are involved in such trouble" .)