The isle in the Bay of Balar, said to have been the eastern horn of Tol Eressëa that broke away when Eressëa was dragged across the sea and anchored near Valinor.

" Ulmo uprooted an island which long had stood alone amid the sea, far from either shore, since the tumults of the fall of Illuin; and with the aid of his servants he moved it, as it were a mighty ship, and anchored it in the Bay of Balar, into which sirion poured his water. Then the Vanyar and the Noldor embarked upon that isle, and were drawn over the sea... But the eastern horn of the island, which was deep-grounded in the shoals off the mouths of Sirion, was broken asunder and remained behind; and that, it is said, was the Isle of Balar. " - From the Silmarillion

The Isle of Balar was used off and on by the Elves from early in the First Age on. It was explored by the Elves of Nargothrond, and Turgon built there havens for the Elves of Gondolin after the Dagor Bragollach.

The Falathrim fled to the Isle of Balar after the fall of Brithombar in the year after the Nirnaeth Arnoediad. Círdan and Gil-galad also settled on the Isle, and it is possible that it was there that Vingilot was built for Eärendil, and from there he sailed to the Blessed Realm.

The Isle of Balar is well known for the abundance of pearls found along the shallow shores which the Elves of Círdan shared freely with the other Elven-lords.

[additional information provided by Ellethiel]