About the Sunburst flag

The sunburst flag is a flag associated with early Irish nationalism, and more recently, youth wings of Irish republican groups such as Na Fianna Éireann. The flag is first thought to have been used in 1858 by the Irish Republican Brotherhood. The sunburst flag is still used today, by both republican groups and the Irish language group Conradh na Gaeilge.

The sunburst flag's design is inspired by the Fianna of Irish mythology. Described as brave warriors who performed a large number of impressive feats, the Fianna referred to themselves as either Gal Gréine or Scal Ghréine, which both mean sunburst.
Regimental flag 2nd Irish Colour, 69th NYSV

The sunburst flag, and the symbol of the sunburst itself came into use by Irish nationalists during the 19th century. In 1858, the Irish Republican Brotherhood adopted the flag as their symbol. During the American Civil War, the sunburst motif was incorporated by several Irish regiments in their standards. In 1893 the Irish-language group called Conradh na Gaeilge established themselves, using the flag as the group's symbol in reference to the Fianna.