You can say it either way, as far as I know. “Sin í an cheist” (ceist is feminine) is a literal translation of “that’s the question”…as if you were answering the question “Cad é sin?” (what is that?), whereas ‘that’s what is in question’ is more what I’d use for this kind of thing. But I think either is technically fine.
I’m definitely open to correction, though! Some of what I’m doing here is uncharted territory for me.
Redwolf
Yeah that was a bit of a 50-50 punt on my part. The fact that an e-fada was available to copy and paste swung it for the masculine choice! 
BTW, any idea how you do fadas when working from a laptop and the right hand numberpad is not available?
Yeah it’s a matter of choice I guess. From memory, I’d tend to go with “Sin í an cheist” in this context but as you say either is technically fine.
If you install the international keyboard layout then you can hold down the right-hand ALT button and press any vowel to get the fada version of that vowel. You can also press the ’ button and then follow it with yo0ur chosen vowel to get the fada version. It has some side effects like swapping the locations of @ and " but works really well for me. Also once you have two keyboard layouts installed you get a little icon on your taskbar to let you easily switch from one to the other.
Thanks for the help avanutria. I installed the US-International keyboard on Regional and Language Options. I’ve got the US-International keyboard selected but still Right (or left) ALT + e selects Edit from the menu bar and typing 'e appears as 'e.
Am I missing something? 
Hm, not sure. It works for me…