From A Wiki of Ice and Fire
The Arbor is an island off the southwestern-most part of Westeros, ruled by House Redwyne. It is considered part of the Reach, the mainland part of which it is separated from by the Redwyne Straits. The port town of Ryamsport is most likely located on the Arbor.
The Arbor is known for making the best wine in Westeros, especially a terrific red one and a gold wine.
Under the high kingship of House Hoare, ironborn rule once extended as far south as the Arbor.
Prince Oberyn Martell and Princess Elia Martell visited the Arbor on their way to Casterly Rock where their mother was to broker their marriage to the Lannister twins.
A Feast for Crows
The Arbor was attacked by the ironborn after the fall of the Shield Islands. Most of the Redwyne fleet was away at the Siege of Dragonstone, and the small fleet left to defend the island was overwhelmed and destroyed. This allowed the ironborn to send their ships up into the Whispering Sound and threaten Oldtown.
References and Notes