Caucasian soldiers initially installed in Irish islands (8)
I believe the answer is:
armenian
'caucasian' is the definition.
(Armenia is a country of the South Caucasus)
'soldiers initially installed in irish islands' is the wordplay.
'soldiers' becomes 'men' (I've seen this before).
'initially' suggests taking the first letters.
'in' indicates putting letters inside.
'irish islands' becomes 'aran' (I've seen this before).
The first letter of 'installed' is 'i'.
'men'+'i'='meni'
'meni' placed into 'aran' is 'ARMENIAN'.
(Other definitions for armenian that I've seen before include "of the Southern Caucasus" , "ethnic group in Asia" , "Person from Yerevan, say" , "language" , "Eg, native of Yerevan" .)