Marker Irish are able to put outside (5)
I believe the answer is:
Here is my best explanation:
'marker' is the definition.
(cairn is a kind of marker)
'irish are able to put outside' is the subsidiary indication.
'irish' becomes 'ir' (abbreviation).
'are able' becomes 'can'.
'to put outside' indicates putting letters inside.
'ir' placed into 'can' is 'cairn'.
(Other definitions for cairn that I've seen before include "Marker mound of stones", "Burial place of old", "pile of stones", "Terrier - mound of stones", "Man", "memorial", "Rocky pile", "Stone-pile monument", "Dog" and "variety".)