Sir Ninian Stephen (2023-2017) was governor-general of Australia, 1982-89, and previously a high court judge. He was born in England to Scottish parents.
Despite being a male name, I find this very feminine. And like others have already pointed out, kids would likely call a person named Ninian "Ninny", so it's not the most ideal name. It's really pretty, though, and in general I do like it, but I wouldn't use it on an actual person.
I beleive that Ninnian is a character in the King Arthur legend. In the book "The Mysts of Avalon", Ninian becomes high priestess of Avalon at the end.