Located in western Minhiriath, Eryn Vorn (likely named so by the Númenóreans) was originally part of the vast ancient treescape that covered most of north-western Middle-earth. During the Second Age, however, these forests were decimated by the Númenóreans in their greed for ship-building timber, before being almost completely burnt down by the forces of Sauron during the ensuing war in Eriador.
By the latter half of the Second Age, the surviving natives of Minhiriath had retreated either north to Bree, or hidden themselves in Eryn Vorn which was probably all that remained of the vast forests in Minhiriath for long years thereafter.
From T.A. 861, Eryn Vorn nominally formed a part of Cardolan and by the end of the Third Age, scattered woodlands had reappeared in much of the rest of Minhiriath.
It is probable that people remained hidden in Eryn Vorn by the Third Age. Although Bree was the westernmost permanent settlement of Men by the late Third Age, the woods of Minhiriath were home of a few, secretive hunter-folk at the time of the War of the Ring. These wild people were afraid to cross the river Baranduin because of the Elves who dwelt beyond.