Sly Green Sky was formed in 2001, originally called Sleeker Synergy. Rooted in hard rock, their sound has elements of world music, flamenco/fusion, jazz, metal, classical, trad, and folk depending on what's in the drinking water. In the early(er) years they played mainly in Dublin in venues such as Wheelan's, Eamonn Doran's, Mother Redcap's and The Voodoo Lounge. The band has since split up but the vocalist and songwriter Craig Ó Mathúna has continued writing and performing and intends to form a new band under the same name using some of the old songs and new songs written since.

So far Craig's lyrics concern a range of topics from universal concepts and issues such as the evolution of human consciousness, human revolution, innate spirituality (aside from organized religion), political satire, as well as one or two songs about more personal and universal issues such as love and passion, and dealing with the psychological roller-coaster of hope and despair that form part of the human condition.