Kind of writing practised by the ancient Irish (5)
I believe the answer is:
ogham
'kind of writing practised by the ancient irish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ogham that I've seen before include "Ancient Celtic alphabet" , "set of characters" , "Old Irish alphabet with letters of lines and notches" , "Celtic characters" , "Ancient British alphabet" .)