The Men of the white Mountains (Dn."Achdael" or "Go-ach") were a number of different, but related peoples during the history of Gondor.
Under the Morachd or King of the Mountains they swore allegiance to Sauron but after his fall became Vassals of the Elendili.When the Dúnedain of Gondor went to the War of the last alliance Elendil the Tall sought support of these men, but they had in secret returned to worship the Dark lord.Due to their oathbreaking these men, under their King Morthec Gruan, became restless spirits, the Shades of the paths of the dead.This Tribe was known as the Daen Coentis.
But years earlier others had fled the mountains or moved to the southern and northern foothills of the Ered Nimrais, these were the Dunir of Andrast and Drúwaith Iaur and the Eredrim of Dor-en-Ernil.Some of their descendants later returned to the white mountains, those were the Orodbedhrim.
But all of these groups were still sometimes collectively known as Mountainmen of Gondor.
The War of the Ring
In the War of the Ring, some of the Mountainmen, especially those from Erech, Lamedon, Lossarnach and Morthond had supported the Steward of Gondor and proved loyalty to Minas Tirith.But other tribes, from the western Ered Nimrais and Haerawreigion, had joined the Dunlendings and Men of Westmarch against the Rohirrim, taking sides with Isengard and the Orcs of the misty mountains.