Some Northern Ireland actors get depressed (8)
I believe the answer is:
downcast
'depressed' is the definition.
(synonyms)
'some northern ireland actors' is the wordplay.
'some northern ireland' becomes 'down' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'actors' becomes 'cast' (group of actors).
'down'+'cast'='DOWNCAST'
'get' acts as a link.
(Other definitions for downcast that I've seen before include "disheartened" , "Sad" , "Feeling despondent" , "Depressed - looking at the floor?" , "blue" .)