Megaera is one from Ireland in the past? Certainly! (7)
I believe the answer is:
erinyes
'megaera is one' is the definition.
(Megaera was one of the Erinyes of Greek myth)
'ireland in the past? certainly' is the wordplay.
'ireland in the past?' becomes 'erin' (poetic or historical name for Ireland).
'certainly' becomes 'yes' (I've seen this before).
'erin'+'yes'='ERINYES'
'from' acts as a link.
(Other definitions for erinyes that I've seen before include "Monsters" , "Fury repeatedly" , "Furies (Greek myth)" , "Snake-haired goddesses" , "Snake-haired creatures" .)