Estermont was conquered shortly after Tarth during the Andal invasion. Peace in the stormlands was eventually achieved when the Durrandons and their First Men bannermen intermarried with the Andals.
Some Storm Kings kept war fleets at Estermont.
A Dance with Dragons
During Jon Connington and the Golden Company's war council at Griffin's Roost it is revealed that Marq Mandrake was put ashore by the Volantenes on what turns out to be Estermont with close to 500 men, although the island was never one of their objectives. Mandrake had no archers with him so Lymond Pease wonders if Greenstone managed to get off any ravens before it fell. Connington answers that he expects they did.
Connington figures there should be ships on Estermont as it is an island. He tells Haldon to send word to Mandrake to leave a garrison behind and bring the rest of his men to Cape Wrath, along with any noble captives. Haldon points out that House Estermont has blood ties with both kings, Stannis and Tommen, so they will make good hostages.