The Brythonic word kombrogos meant ‘land of the compatriots’ (brogi meaning territory) and this is believed to be the source of the term Cymry which still endures today in the country we know as Wales (see this post on why it was later named ‘Wales’)
Cymry (pron. cum-ree) referred to both the people and their land up until the 16th C. After that, the phrase Cymru (ie: the land of the Cymry) also came into use to describe the land but not the people. Unhelpfully, the two words are pronounced identically and its only context that creates the distinction.
Cymry and Brythoniad appear to be interchangeable in ancient Welsh literature referring to those people who inhabited the vast island prior to and immediately after the Romans but before the Anglo-Saxons swept through in the 6th century.
Because ‘y Ddraig’ is set in the sixth century, I have used the authentic ‘Cymry’ to describe the people and their lands, and I have used Brython to describe the greater island that those lands were in.
Cambria (pron. cahm-bree-ya) and Cumbria (pron. cum-bree-ya)
The Romans referred to the lands in Britain’s south-west as Cambria and in the north as Cumbria (note the very suble a/u vowel shift) while the lands in the middle were Cymry. The north came to be known as Cumberland and the people in it the Cumbers.
Thus, we have a vast area of post-Roman Britain being called (phonetically… You say it and see how long the ‘b’ lasts) Cum-ree, Cum-ree-a, or Cam-ree-a. This map at the turn of the 5th/6th century bears that out. The borders of the Cymry (shown in the Project Gutenberg map below as the Anglo-Saxon name ‘Welsh’) shifted and changed throughout the 4th, 5th and 6th centuries depending on where the people within them fled to or how hard they held on against invasion. The northern, southern and eastern parts of Cymry were subsumed by the Angles first and the west held on the longest. But not forever.
At the end of the 7th century, the Anglo-Saxons had overthrown the land known as Cumberland (the land of the Cumbers) and referred to it as North Cumberland (today’s Northumberland). The fact that Cumberland had a North means it also had a south (Cymry) but those lands were not the Anglo-Saxons to name for quite some time.