Eagles Have Flown
Antonius Donatus Gregorius
King of South Wales
Antonius, also known to Welsh tradition as Anawn Dynod, was another son of Magnus Maximus, set up in his own territorium by his father or shortly after his departure. The positioning of Demetia, kingdom of the Demetae Britons, meant that it could protect the south west coastline from Irish raids.
The South Wales region soon crystallised into the kingdom of Demetia, later Dyfed. and was made up of two main regions. The larger part of the territory was Dyfed itself, and Ystrad Towy (or Towi) made up the eastern third or so. This was conquered in around 690 by Ceredigion, but Wales was united under the kings of Gwynedd in the ninth century, and Dyfed came to form the heart of a united South Wales after that, continuing through Deheubarth. The later kingdom of Brycheiniog also seems to have (at least partially) formed part of the South Wales territory.
As a complete opposite of the process going on in North Wales at the same time, Irish settlement in Demetia was actively encouraged, in the form of inviting the Irish Déisi tribe to settle there. They seem to have been brought in to act as foederati on the west coast, keeping the British shores clear of Irish raiders, an apparently successful policy. In the region, many Irish words are mixed into the regional dialect, and there is a considerable spread of memorial stones pointing to Irish influence. The incidence of Ogham (Irish) symbols, highest in the modern county of Dyfed, is a crude guide to the settlement of the Déisi, and after the British line of rulers had died out, the Déisi were best placed, through intermarriage, to assumed the legitimate rule of Dyfed.
from Kessler, P. L. (2007, September 8). Post-Roman Britain: Early Independent Britain AD 400-425. Retrieved from http://www.historyfiles.co.uk/FeaturesBritain/BritishMapAD400.htm