Though referred to as mountains, the Mountains of Mirkwood were actually a range of tall hills running through the Forest of Mirkwood in an east-west line, to the north of the Old Forest Road.

The Enchanted River, a tributary of the Forest River , crossed by Thorin and his company in TA 2941, had its source in the Mountains of Mirkwood.

When the Sylvan Elves, led by King Thranduil first settled in Mirkwood the mountains were referred to as the Emyn Duir, but after evil creatures began multiplying in the forest, the name was change to Emyn-nu-Fuin.

When the Evil was cleansed from Mirkwood and it was renamed the Wood of the Greenleaves (Eryn Lasgalen), the Mountains of Mirkwood became the boundary between the Kingdom of the Wood-elves and the land of the Beornings and Woodmen.
Encyclopedia entry originally written by PotbellyHairyfoot