Ancient people in Dublin Castle (5)
I believe the answer is:
incas
'ancient people' is the definition.
(people of the Inca Empire)
'in dublin castle' is the wordplay.
'in' indicates a hidden word.
'INCAS' is hidden within 'dublin castle'.
(Other definitions for incas that I've seen before include "Old South American royals" , "Old people coming from South America" , "People from Andes who settled in Peru" , "Old Peruvians" , "Ancient inhabitants of Peru" .)