Seren is the modern Welsh for a star. No Gaulish name could be 'derived' from it.
Sirona, if it existed in Modern Welsh (and it doesn't), would, due to the normal sound changes, look more like Hiron (and I think the vowels would change too).
For the pronounciation, all you really need to know is whether the vowels are long or short - it could be either sirowna or sironna... But you'd have to find the original record for that.