Estermont

Estermont is a mountainous little island in the Narrow Sea in the Stormlands. The island is ruled by House Estermont.

Season 5
While traveling to Dorne, Ser Jaime Lannister ask if the island they are sailing pass is Estermont, but the ship's captain informs him it is Tarth.

In the books
In the A Song of Ice and Fire novels, Jaime Lannister has previously visited Estermont. King Robert Baratheon's maternal uncles came for his and Cersei Lannister's wedding and stayed for six months and Robert insisted on returning the courtesy with a two-week visit to Estermont. Jaime follows Robert one night confirming Cersei's suspicions that Robert was having sex with his widowed cousin. Jaime asks Cersei if she wanted Robert dead, but instead Jaime and Cersei have sex beginning their long secret incestuous extramarital affair.