Author: Aneirin, aka Neirin or incorrectly Aneurin, a Cumbric bard. He was attached to the Celtic chieftain Urien, who ruled in Rheged (now Cumbria).
Where: Somewhere in Y Hen Ogledd (The Old North). These were once the most northern lands occupied by the Brythonic Celts, until the Angles over-ran it. Parts of Yorkshire, Durham, Tyne and Wear, Northumbria and Cumbria in England, plus the Lowlands of Scotland, collectively make up the geographical region of Y Hen Ogledd.
Cases have been made for Dumbarton (Scotland), or West Yorkshire (England) as being Aneirin's homeland.
Din Eidyn, mentioned in the poem, is almost certainly Edinburgh.
Story told: The bard was an eye-witness at the catastrophic Battle of Catraeth, emerging as just one of four survivors. He penned Y Goddodin as a series of interlinked elegies, telling the story of the battle through those who died in it.
Aneirin does not claim that Arthur was there. He's mentioned as a comparison for another man, Gwawrddur. The 99th stanza ends with the lines:
He fed black ravens on the rampart of a fortress
Though he was no Arthur.
Among the powerful ones in battle
In the front rank, Gwawrddur was a palisade.
Possible agenda: The mention of Arthur is blatantly a cultural reference, which Aneirin expected his audience to understand. It helped them to quickly grasp a sense of what Gwawrddur was like as a warrior and a man; a short-hand code, conveying volumes, without the bard needing to spell it out.
Like me saying, "The boxer was no Mike Tyson, but he was handy with his fists."
The familiar name also lent a veneer of sympathy for the strange warrior. They might not know Gwawrddur, but they knew Arthur, and the warrior was like him. Their feelings could be transferred.
Aneirin proved himself a master of this story-telling device by taking it a stage further. 'Gwawrddur was an Arthur' would have worked as an emotive analogy, but it becomes much more beguiling to say 'Gwawrddur was no Arthur'. Now the warrior seems flawed and human, just like us, but why was he 'no Arthur'? Our imaginations work to fill in the details, guessing at the gap, fixing an ever more vivid impression of Gwawrddur in our heads.
Which all serves to make his loss on the battlefield instantly more tragic. The bard made us care about the unknown soldier.
Finally, Aneirin's agenda - in referencing Arthur - could be nothing more complicated than formatting. In the 6th century, Celts heard their stories, legends, histories and poems through the medium of their bardic oral tradition. It's not obvious in English translations, but the lines of each Y Goddodin stanza end in rhymes. That grouping served as a mnemonic for the bards reciting it.
Gocharai brain du ar fur caer
Cyn ni bai ef Arthur.
Rhwng cyfnerthi yng nghlysur,
Yng nghynnor, gwernor Gwawrddur.
In the Welsh alphabet, 'dd' is the equivalent sound for 'th'. Gwawrddur may have been selected for the analogy simply because his name rhymes with Arthur.
Addition to the Arthurian legend: Arthur had become the benchmark for steadfast bravery, against which all Celtic warriors might now be compared.
Plus he was famous enough for his name to be used as an analogy, in the full expectation that all audiences would have a frame of reference. This can only mean that he was a) a Celtic deity; b) a popular hero from legend; or c) an extremely famous historical or contemporary individual.