Comments for the name Donovan

I have a book of names that says it means "Dark Warrior".

Why is it from Ó Donnabhain? Shouldn't they just say it's from Donnabhain?

Great name, though. Very masculine and cool. And it is certainly NOT the only Irish name that is so.

There should be an accent on the surname version: Ó Donnabháin. I've never met an Irish-speaker with this first name, but if I did, I would expect it to be spelled Donnabhán in that case, as Donnabháin is the genitive, 'of Donovan'.