The island kingdom destroyed before the founding of Gondor in J.R.R. Tolkien’s legendarium is Númenor.
Númenor is an island realm granted to the Men who aided the Elves during the War of Wrath at the end of the First Age. Its people, the Númenóreans, receive long life and great maritime skill, but they remain mortal. Over time, fear of death and resentment toward the immortal Elves grow among the kingdom’s rulers.
The king Ar-Pharazôn eventually sails against Valinor after being deceived by Sauron. The Valar call upon Eru Ilúvatar, who changes the shape of the world and causes Númenor to sink beneath the sea. Elendil and his sons escape the catastrophe.
Elendil establishes the realms of Arnor and Gondor in Middle-earth. Númenor is also called Westernesse in some English renderings, but that name refers to the same legendary island kingdom.