Complete collection of religious books discovered in Ireland (6)
I believe the answer is:
entire
'complete' is the definition.
('entire' can be a synonym of 'complete')
'collection of religious books discovered in ireland' is the wordplay.
'collection of religious books' becomes 'nt' (New Testament).
'discovered in' is an insertion indicator.
'ireland' becomes 'eire' ('Eire' can be a synonym of 'ireland').
'nt' going into 'eire' is 'ENTIRE'.
(Other definitions for entire that I've seen before include "Whole and complete" , "Whole; absolute" , "Utter" , "Whole; unbroken" , "excluding nothing" .)