Pindar (522-443 BC) was one of the nine ancient Greek lyric poets. He refers to the ‘Pillars of Heracles’ in three of his victory odes, Olympian 3.43-45, Nemean 3.19-23 and Isthmian 4.11-13, in a manner which indicates that the term was used as a metaphor for the limit of human (Greek) experience. As such, the location of the Pillars would have changed as the colonial and commercial expansion of the Greeks developed beyond the Aegean.
Oliver D.Smith has drawn attention to the Nemean Ode 4, 65-70 as having echoes of Plato’s description in it(a).
(a) http://atlantisresearch.wordpress.com/ (now offline)