The Lord of the Rings: Shadow of the East
The most ancient native people of Siregalë are the Sirannim, a tribal grouping of Silvan Elves.
The ancestors of the Sirannim were Nandor, the Moriquendi who, in the Age of the Stars, turned away from the march to Aman to settle among the woodlands of the lower Anduin. In a later time, a host of the Nandor migrated westward, probably fleeing nameless evils then arising in the land of Mordor, to settle among the flanks of the Blue Mountains in Ossiriand (Lindon). Here they were known as the Laegrim (S. “Green-elves”). Boromir was their King in the Elder Days, but he died, ere the first rising of the sun, at the hands of the Orcish minions of Morgoth.
The Laegrim then made alliance against Morgoth with the Noldor and Sindar of Beleriand, but had no King of their own. The Sirannim, the scattered clans dwelling east of the mountains, were far from any Sinda or Noldo lord; their resistance to the Black Enemy was led by Merethorn, one of their clan chieftains. After the fall of Beleriand, Siragalë was considered a demense of the Elvish kingdom of Lindon. Merethorn became ruling Lord of the Sirannim, marrying a Sinda Lady (of part Noldo descent) to seal his oath of loyalty to Gil-galad, High King of the Noldor.
The fate of the Sirannim then became tied to the slowly declining fortunes of the western Elves. Gil-galad’s kingdom lasted for the entirety of the Second Age of Middle-earth, and Merethorn fought loyally in his wars. He died in the last of them, on the sands of Mordor during the siege of Barad-dûr. His widow, Ellindiel, assumed the rule of Siragalë, tending to the affairs of a diminished and bitter people.
After Gil-galad persihed at the hand of Sauron on the slopes of Mount Doom, Ellindiel refused to swear allegiance to his successor, the Sinda Lord Círdan of Lindon. Instead, she joined the fate of the Elves of Siragalë to the Dúnadan kingdom of Arnor. When Arnor was divided in T.A. 861, Ellindiel cast her lot with the successor kingdom of Arthedain. The Sirannim retained their autonomy throughout the history of both realms, rarely fighting in mannish wars.