The phrase "fire in the head" is a Celtic metaphor for divine inspiration. It ultimately derives from the "Song of Amergin," but has cognates in the poetry of Taliesin, whose very name means "Shining Brow."

It also appears in W.B. Yeats' poem "The Song of Wandering Aengus":

I went out to the hazel wood,
Because a fire was in my head.