I think that, thanks to you, it has grown on me! I wouldn't use it myself but I think it's pretty without being overly frilly.
I've never met an Oona. I think I prefer the Oona speling. Una reminds me of Uno (the game) and Oonagh may be difficult to spell.
Is Oonagh the traditional spelling?
Oonagh is one of the Irish versions (also Úna), but I've never really been one for being hyper-correct about Irish names. I agree that the extra consonants don't really add anything.
Oona is an acceptable variant that is also used in some Scandanavian countries.
I fell in love with it because I was/am a huge Anne of Green Gables fan — a character in the later books was named Una. So I originally preferred that spelling, but I've grown to like Oona. I sort of like that it's a little oddball, but it's still a nice, soft, vowelly name.