Vita Merlini--The Life of Merlin--is a Latin work which claims to be authored by Geoffrey of Monmouth, better known for his work The History of the Kings of Britain, from which we first hear of Merlin, who was wizard and advisor to Uther Pendragon and, in later romance, Uther's son King Arthur.
The poem tells of how Merlin went mad after the battle of Arthuret, and how his sister Guendoloena and the bard Taliesin try to restore his sanity. As such, it holds much to the stories of Mad Sweeney and Lailoken.
The early Myrddin poems seem to be analoguous to some of the action in this piece--in both The Black Book of Carmarthen and the Red Book of Hergest, there are poems attributed to Myrddin, where he covers much of the same topics with Taliesin and his sister, here called by the Welsh name Gwenddydd.
My noding--editing, really--is a work in progress.
Gaufridi de Monemuta
The Life of Merlin
By Geoffrey of Monmouth
translated by Basil Clarke, edited by me.
University of Wales Press