It's not even remotely Celtic in origin. X doesn't exist in any of the modern Celtic-language alphabets, silent or otherwise, and C is normally pronounced like the c in 'cat'.
Cenedra, sans the x, has however been used 4 times in the UK, and all of those usages date after the publication of a fantasy series called 'The Belgariad' by David Eddings, in which, according to Wikipedia, there's a character called 'Ce'Nedra' who alternatively goes by ''X'nedra':
https://en.wikipedia.org/wiki/List_of_The_Belgariad_and_The_Malloreon_characters#Protagonists