Fionn’s window is a mandalah-like diagram found in the Book of Ballymote, a 14th century Irish manuscript of bardic lore. This window is composed of letters of the Ogham alphabet arranged on a set of five concentric circles.
Associated with the lore, or legend, is the legendary Irish hero Fionn mac Cumhall, also credited with many other elements of bardic lore, and is certainly an archetypal. Fionn’s window has been resurrected by some modern Druid and Celtic groups. A.G.H.
Greer, John Michael. The New Encyclopedia of the Occult. St. Paul. MN. Llewellyn Worldwide. 2005. p. 174