This name appears briefly in a poem in the Black Book of Carmarthen in the form Kyrridven [1] and in a poem in the Book of Taliesin in the form Kerrituen [2]. Some theories connect her to an otherwise unattested Celtic goddess of inspiration, and suppose her name is related to Welsh cerdd "poetry".
The Cornish form of this name, Jennifer, has become popular in the English-speaking world.
Geoffrey based parts of Merlin's character on Myrddin Wyllt, a legendary madman and prophet who lived in the Caledonian Forest. Other parts of his life were based on that of the historical 5th-century Romano-British military leader Ambrosius Aurelianus (also known as Emrys Wledig). In Geoffrey's version of the tales and later embellishments Merlin is a magician and counselor for King Arthur.
As an English name, it became popular due to the Fleetwood Mac song Rhiannon (1976), especially in the United Kingdom and Australia.