As an English name, it became popular due to the Fleetwood Mac song Rhiannon (1976), especially in the United Kingdom and Australia.
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.
The Cornish form of this name, Jennifer, has become popular in the English-speaking world.
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".