Rocky island in southern Irish county (6)
I believe the answer is:
skerry
'rocky island' is the definition.
(I've seen this before)
'southern irish county' is the wordplay.
'southern' becomes 's' (abbreviation).
'irish county' becomes 'kerry' (I've seen this before).
's'+'kerry'='SKERRY'
'in' is the link.
(Other definitions for skerry that I've seen before include "tiny island" , "small island" , "Rocky island or reef" .)