Eilenach was the second of the seven beacon-hills of Gondor, after Amon Dîn in the east, and it was the highest point of the Drúadan Forest. It had a sharp summit, uncapable of sustaining a large fire. Its main purpose was to relay the fire of Amon Dîn to Nardol.
On the origin of the name, Tolkien writes Eilenach is "probably an alien name, not Sindarin, not Númenórean, or Common speech." Perhaps it was pre-Númenórean
The Lord of the Rings: A Reader's Companion, page 510.