What's the difference/when would you use 'Agus cén uair' vs 'Agus cá huair', since both are accepted as 'and when?' here?
That seems odd, as cén means what, so "Agus cén uair" literally means "And at what time", whereas cá means where, so "And where's the time" doesn't make much sense to me, at least.
"cén uair" and "cá huair" are synonyms and the difference in use would apparently be one of either dialect or personal preference.
With reference to PaulCullory, these are probably rather idioms and not literal translations:
If you scroll down the page in the link to the samples in alphabetical order then both questions are shown.
Cá huair? At what hour? When?
Cén uair? At what time? When?