Poster in Northern Ireland supported by Republican is low point (5)
I believe the answer is:
nadir
'low point' is the definition.
(I've seen this before)
'poster in northern ireland supported by republican' is the wordplay.
'poster' becomes 'ad'.
'in' means one lot of letters goes inside another.
'northern ireland' becomes 'ni'.
'supported by' is a charade indicator (letters next to each other) (letters go below - in down clue).
'republican' becomes 'r' (abbreviation).
'ad' put within 'ni' is 'nadi'.
'nadi'+'r'='NADIR'
'is' is the link.
(Other definitions for nadir that I've seen before include "extremely low position" , "Lowest point, or time of deepest depression." , "pits" , "Rock bottom" , "Drain (anag)" .)