Being fluent in Welsh and having several Ceri's at the school which I attend, I would like to point out that Ceri can NOT be pronounced Serry - only like Kerry.
I agree it can only be pronounced 'Kerry'. The Welsh 'C' is only ever pronounced 'K'. If you want your child to be named 'Serry', name them Serry. Not difficult.
This is my name; it's not bad being called Ceri, although if you say it aloud everyone thinks its spelled Kerry, and if you write it down people think your name is said SERRY.
I've never met another person called Ceri - apparently this is important to parents, although I don't really understand why. It's not like I spent my childhood going, "Wow another day at school being the only kid called Ceri; what a freaking thrill".