When do you use "Cén uair" vs. "cathain"?
In the case, where you ask "when" like in "what day of the week", then "cén uair" is not perfectly suited, as that question form would rather expect an answer related to time on the clock.
I wrote "What reason and when." I think that "what reason" is synonymous with "how".
Will it accept 'What reason and place?'?
Maybe? Though I'd argue cén fáth just means 'why now
So what is just Why? Just out of interest?
There is no single word equivalent for the adverb “why”, but there’s a choice of equivalent phrases.
Is cathain pronounced the same as caitheann?
"Slender with slender & broad with broad". The i/e combo makes the second word closer to "ca-hyin".