*Outigirn is a name that appears in HG 10. But in JC 5 his name has been corrupted to *Kyndeern weldic.
A later individual with the same name is to be found in HB 62:
Then Dutgirn at that time fought bravely against the nation of the Angles. At that time, Talhaiarn Cataguen was famed for poetry, and Neirin, and Taliesin and Bluchbard, and Cian, who is called Guenith Guaut, were all famous at the same time in British poetry.
The document indicates that *Dutgirn or *Outigirn (Eudeyrn) lived in the time of Ida and Maelgwn. The corruption of the name Eudeyrn to Cyndeyrn that occurred in JC 5 may have occurred in the opposite direction, i.e. Cyndeyrn to Eudeyrn in the HB. Chronology suggests the HB reference is to Cyndeyrn Fendigaid ab Owain ap Cyngar who appears in ABT 18a and JC 13. He was not Cyndeyrn Garthwys ab Owain ab Urien Rheged of ByS 14.
The name Macsen Wledig has been applied to two distinct persons. The first individual appears in gen. -8 of HG 2 as Maxim gulecic, see Why Bartrum’s dating …. I believe this individual can be identified with the Roman emperor Constantius Chlorus as explained in the above link.
The opening lines of The Dream of Macsen Wledig from Pen. 4. (The National Library of Wales)
The Pen. 16 version of The Dream of Maxen Wledig only describes the story up to his stay in Britain. The version in Pen. 4 of the White Book of Rhydderch speaks about his return to Rome, but this continuation is actually referring to the second Macsen.
This is the Western Roman emperor Magnus Maximus born c. 335. He appears in gen. -5 as in the follwing fragment from JC 13.
Cyngar m. Protec
Cyndwr Fendigaid is St. Kentigern who was the son of Owain ap Cyngar and not Owain ab Urien Rheged. The table is in accord with Constantine the Great’s birth in 272 or 273 and the birth of Helena, a wife of Constantius Chlorus, birth between 248 and 250.