Our most recent post in our monthly topic focused on the character Merlin, and in particular the form of his name given to him by Geoffrey of Monmouth: Merlinus Ambrosius. Having concentrated on the Merlin/Myrddin forms in that post, in this one we now take up the issue of Ambrosius, and its connections to the Welsh name Emrys.
Monmouth is the first to give Merlin the epithet Ambrosius, and this element is distinctly non-Celtic: It is a Roman nomen ultimately deriving from Greek ἀμβρόσιος ‘immortal, divine’. The most famous historical Ambrose is the 4th C church doctor Aurelius Ambrosius, better known modernly as St. Ambrose of Milan. While Ambrose was never a popular name, it was used throughout Europe. The use of the name almost certainly is due to the fame of the saint, and not due to the Arthurian connections via Monmouth.
Merlin is not the only Ambrosius to appear in connection with the Arthurian myths, nor is St. Ambrose the only historical Ambrosius who has a second name associated with the word aurelius. In the 6th C De Excidio et Conquestu Britanniae, the historian Gildas mentions a 5th C Romano-British war leader Ambrosius Aurelianus; in Welsh histories, he is known as Embreis Guletic (Guletic being a form of gweldig ‘lord’). A 9th C Historia Brittonum, often attributed to Nennius, confuses this person with the boy who became Merlinus, resulting in Monmouth’s conflation of the names into Merlinus Ambrosius .
So what about this Welsh form of the name, Emreys or Embreis in Old Welsh and Emrys in modern Welsh [2,3]? We have no non-literary examples of the name used by people in the Middle Ages. It does show up in a place-name associated with the Arthurian character: Dinas Emrys, a post-Roman hillfort where, according to the Historia Brittonum, a young Merlin prophesied to Vortigern that the white dragon of the Saxons, would be conquered by the red dragon of the Welsh [*].
References & Acknowledgements
 Hutson, Arthur E., British Personal Names in the Historia Regum Britanniae (Berkeley: University of California Press, 1940), pp. 58, 119.
 Bartrum, Peter C. Welsh Classical Dictionary (Aberystwyth: National Library of Wales, 1993).
 Bromwich, Rachel. Trioedd Ynys Prydein. (Cardiff: University of Wales Press, 1978).
[*] Many thanks to Dr. Heather Rose Jones for sharing her research on the medieval usage of Emrys.