Brasil, also known as Hy-Brasil or several other variants,is a phantom island which was said to lie in the Atlantic ocean west of Ireland. In Irish myths it was said to be cloaked in mist, except for one day each seven years, when it became visible but still could not be reached. It probably has similar roots to other mythical islands said to exist in the Atlantic, such as Atlantis, St. Brendan's Island and the Isle of Mam. The etymology of the names Brasil and Hy-Brasil are unknown but in Irish tradition it is thought to come from the Irish Uí Breasail (meaning "descendants (i.e., clan) of Breasal"), one of the ancient clans of northeastern Ireland. cf. Old Irish: Í: island; bres: beauty, worth; great, mighty.

Hy-Brasil was originally thought to be the origin of the name of the modern country of Brazil. However, the generally accepted theory now states that Brazil was named for the brazilwood, which has an extreme red color (so "brasil" derived from "brasa": ember), a plant very valuable in Portuguese commerce and abundant in the new-found land.