Northern Irish county; in low spirits (4)
I believe the answer is:
down
I believe this clue is a double definition.
'northern irish county' is the first definition.
(County Down in Northern Ireland)
'in low spirits' is the second definition.
(thesaurus)
(Other definitions for down that I've seen before include "like the other set of clues" , "See 18" , "Not working (computer)" , "Depressed; swallow" , "In a low mood; hill" .)