Legendary Irish hero and bard of the 3rd century AD (6)
I believe the answer is:
ossian
(Other definitions for ossian that I've seen before include "Gaelic bard, son of Fingal" , "Invented Irish hero" , "Legendary Gaelic poet faked by Macpherson" , "Legendary Irish poet invented in eighteenth century; is a son (anag.)" , "Legendary Cthree Gaelic bard" .)