"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?