Merlin is the form in which we take the enchanter's name from Norman French.  In Welsh it is Merddhin, and the Triads tell us of the three baptismal bards of Britain,—Merddhin Emrys, Merddhinn ap Madawg Mororyn, and Taliessin.  There were also three disappearances from Britain, those of Gavran, of Madawg, and of Merddhin Emrys, who, in Welsh story, went off, not with Vyvyan, but with nine bards in a ship of glass, to the happy islands of the West.  As to the poems and prophecies current in Merddhin's name, they are beyond computation.
    M. de Villemargqué has compiled the narration of which the following is an outline.  He thinks that the original idea is to be found, by going back to the Marsi, ancient inhabitants of Apulia, who were great physicians, and supposed to derive arts of magic from their god, Marsus; and thus, that among the Romans, Marsus came to be synonymous with a magician.
    The Britons and Armoricans, in their Romanized state, came, he thinks, to use the same term, only pronouncing it Marzin and Marddhin.  Leaving some of the Roman deities, whose altars were multiplied all over Britain, and of these, more to the obscure and local deities who were tutelary to individuals and nations, than to the great Olympian divinities; the Armorican Cymry came to make of Marzin a sort of god, with three kingdoms of flowers, golden fruits, and of laughing pygmies.
    He further thinks that Emrys, or Ambrosius, was really a young bard, who grew up at the court of; the great Ambrosius, and who was baptized by the same name, though called Merddhin from his talents, and perhaps his relapse into heathenism.  With Gwrtheyrn, there may have been a sort of revival of Druidism, of which Merddhin was probably the leader; some fresh consecration of Stonehenge, and a renewal of ancient rites, calling forth the vehement censure of Gildas, for it seems that Gospels were torn, churches burnt, and monasteries robbed.  He is thought, however, to have lived through Arthur's reign, and then, after the fatal battle of Camelford, to have poured forth lamentations in solitude, much like that of Ossian after the Feen, until he was consecrated, the Scots say by St. Kentigern, the Irish by St. Columbanus, the Bretons by St. Cadoc.
    The person, however, who wrote the lamentations here referred to, may have been the second Merlin in the Triad, also called Merddhin the Caledonian, or Merdhinn Vardd, or Merddhin ap Morvryn.  According to Davies, Merddhinn and Morvryn are the same name, and both mean hill in the sea; and he explains Merddhin Vardh, as bard or priest of the sea-girl hill. 
    Whether he is right or not in so explaining the origin of the word, Merddhin is in sound Mervyn, and this, as well as Marzin, is popularly used for the great magician in Brittany, instead of the Merlin of French and Latin romance, or Merlino of Italian... (History of Christian Names, Yonge, v.2, 1878)