While I was going through Old Irish names and words, I came across this very interesting definition:
Then based on this annotation:
Dathí
Already in the 11th century the meaning and origin of the name had become obscure to the Irish grammarians. It was associated with daith, daithe, swift, swiftness (and the npr. Daithe, Daithem, Daithen) as is clear from the marginal note LU 38.
And the history given in
The Genealogies, Tribes, And Customs Of Hy-Fiachrach:
Dathi went afterwards with the men of Erin across Muir n-Ichtᵛ towards Leathaʷ, until he reached the Alpsˣ, to revenge the death of Niall of the Nine Hostagesʸ.
...
Dathi came to the tower. (He was called Dathi from his expertnessᶜ [daithe] at invading [evading?] and shooting, for if there were one hundred persons shooting, i. e. discharging arrows or javelins at him, he would be protected against them by the activity of his hands in guarding, wherefore the name of Dathi clung unto him. Feradhachᵈ was his name when he went to the east, and it was on his expedition in the east he was called Dathi.)
I think from all the above that Dathí was originally named
Feradach Daithe, or Feradach the Avenger, NOT Feradach the Swift. So his name was shortened to his epithet / cognomen Avenger, NOT Swifty. That makes a lot more sense in context of the history. Otherwise, it makes him out to be like one of those ridiculous Chinese action movie heroes who deflect swords, arrows, and spears with their bare hands.
As the annotation indicates, the name Dathí / Nathí became hopelessly confused and conflated over the centuries. Our ancestors were not stupid men and I believe in giving them the benefit of the doubt, despite later silly misinterpretations.
