Seán __, Irish playwright long associated with the Abbey Theatre, Dublin (6)
I believe the answer is:
ocasey
(Other definitions for ocasey that I've seen before include "Sean . . . . . ., great Irish playwright of 'The Plough and the Stars' (1'5)" , "Sean .' ....., author of ` `The Silver Tassie' ' (1'5)" , "Sean . . . . . . wrote 'Juno and the Paycock'" , "one connected with The Abbey" , "Sean ..., author of 'The Plough and the Stars'" .)