The Pass of Sirion led into Beleriand from Ard-galen. The pass contained the River Sirion and was bordered on the east side by the Dorthonion plateau, and on the west side by the Ered Wethrin.

The Pass of Sirion was of extreme strategic importance for the Noldor in the First Age, as it was the cornerstone of their western defences against Morgoth's realm in the North. Finrod built a tower on the island of Tol Sirion, in the middle of the river, and this further increased its defensive power.

Sauron took the Pass during the Dagor Bragollach, but it was retaken preceding the Nirnaeth Arnoediad.