The island of Hy-Brasil was often shown on maps as a very small island west of Ireland and originated from Celtic mythology.
As early as the 12th century, the Irish believed in the existence of an enchanted island that could be viewed along the west coast of Ireland once every seven years. Hy-Brasil continued to appear on maps until 1873 when it was shown for the last time on a British Admiralty Chart.
There was an oral tradition of telling stories about an island in a fog bank or a floating island that would disappear when people approached it.
The generally accepted theory states that it was renamed for the brazilwood, which has an extreme red color (so brasil derivates from brasa: ember), a plant very valuable in Portuguese commerce and abundant in the new-found land.
The most distinctive geographical feature of Hy-Brasil is that it appears on maps as a perfect circle, with a semi-circular channel through the center. The central image on the Brazilian flag, a circle with a channel across the center, was the symbol for Hy-Brasil on early maps.
One of the most famous visits to the island of Hy-Brasil was in 1674 by Captain John Nisbet of Killybegs, Co. Donegal, Ireland. He and his crew were in familiar waters west of Ireland, when a fog came up.
As the fog lifted, the ship was dangerously close to the rocks. While getting their bearings, the ship anchored in three fathoms of water, and four crew members rowed ashore to visit Hy-Brasil. They spent a day on the island and returned with silver and gold that was given to them by an old man who lived there.