Demons one youngster encountered in Northern Ireland (6)
I believe the answer is:
incubi
'demons' is the definition.
(incubus is a kind of demon)
'one youngster encountered in northern ireland' is the wordplay.
'one' becomes 'I' (Roman numeral).
'youngster' becomes 'cub' (I've seen this before).
'encountered in' is an insertion indicator.
'northern ireland' becomes 'ni'.
'cub' inserted within 'ni' is 'ncubi'.
'i'+'ncubi'='INCUBI'
(Other definitions for incubi that I've seen before include "Demons of the night" , "nightmares" .)