From A Wiki of Ice and Fire
Tarth is an island off the main continent of Westeros that is the fief of House Tarth, who rule from Evenfall Hall. It is separated from the continent by the Straits of Tarth, and is situated northeast of Shipbreaker Bay. Tarth is considered a part of the Stormlands.
The island is said to be very beautiful, "with its mountains and waterfalls, its high meadows and shadowed vales." It is called the "Sapphire Isle," for the striking blue seas in which it sits.