Some Northern Irish players unhappy (8)
I believe the answer is:
downcast
'unhappy' is the definition.
(I know this)
'some northern irish players' is the wordplay.
'some northern irish' becomes 'down' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'players' becomes 'cast' (group of actors).
'down'+'cast'='DOWNCAST'
(Other definitions for downcast that I've seen before include "Sad" , "Feeling despondent" , "blue" , "Dejected and discouraged" , "Dispirited, gloomy" .)