From A Wiki of Ice and Fire
Brandon the Shipwright, King in the North, sailed across the Sunset Sea and was never heard from again. During the reign of Qhored Hoare, King of the Iron Islands, the ironborn controlled the western shore of Westeros, but his gains were gradually lost by his descendents. Lord Dagon Greyjoy attempted to make the lands along the sea subject to the ironborn again, but was eventually defeated by House Targaryen.
A Feast for Crows
During the kingsmoot, Lord Gylbert Farwynd puts his name forth for King of the Iron Islands, supported by three champions who are his sons. He promises to lead the ironborn to lands beyond the Sunset Sea where every man would be a king, but his claims are quickly dismissed.
|“||No one has ever crossed the Sunset Sea to learn what lies on the other side.||”|