It’s about as close to a random person as you can get while still being recorded. They were royalty, but the two real ones get literally a sentence each at max
Eithne ingen Bresail Bregh married the king of Tara and is described as “having deserved reward from God for her good works, and for her intense penance for her sins” in one source and “deserved to obtain the heavenly kingdom, having done penance” in the other
Eithne ingen Cinadhon was the daughter of a Pictish king and is literally only recorded as having died
The legendary Eithne is the daughter of a king of Scotland (mostly Pictish at the time) and crossed the sea to Ireland, where she gave birth to the hero Túathal Techtmar. This is the entirety of her role in the story; a couple of paragraphs in a collection that, in the translation I’m looking at, has 600 pages just for part five
It’s about as close to a random person as you can get while still being recorded. They were royalty, but the two real ones get literally a sentence each at max