The old Irish had many stories about sailing west and finding islands there; Tír-Na-nÓg, the Isles of the Blessed, St. Brendan's Isle... Hy-Brasil is one of these, an isle of mists, coming and going in the fog. It is somewhat unusual in that it supposedly required no long journey to reach (there is a tenacious pseudotheory that St. Brendan's Isle was Canada): instead, it is typically placed on maps just outside Galway Bay. In consequence, Brazil is probably not connected to Hy-Brasil; the names say the same, for Í Breasail is the Isle of the Beautiful or of the people of the wretched king Bres, whereas Brazil is named for that treasure of the Amazon, brazilwood.
In some stories, the island appears every seven years, visible for a day and a night, and men may then attain it; in others, the island when revealed is still a mirage. Yet, somehow, belief in the island stayed; there are purported witnesses across the ages, who are to have seen the island appear or stood upon it themselves; there is an extant book which is said to have been brought enchanted from its shores; it was regularly marked on maps even in the 19th century; in times closer to our own, there have been earnest speculations that it is a rock or reef which is submerged except in certain rare conditions of the weather and climate. Which is a crock, of course, but fairy-tales sometimes are hard let go.
All this, and I can still not tell you if they sleep aloft in Hy-Brasil; all I know is the age of Uqbar. I have let you down.