Anyone know if this would be considered impolite in most situations, as it would in English?
I wouldn't consider it polite, it's pretty much the same as being asked "What's your name?" or "What are you called?"
I agree, I'd use the informal unless I know for certain that it's someone older than me or someone like a teacher.
What's the difference between còt thusa, cò sibhe and ... the third one. Was it cò thu? Is it singular, plural and formal? But which is which?
Only 2 forms.
Cò thusa? is informal and singular.
Cò sibhse? is formal or plural
How do I get accents on my letters?
On a phone, pressing and holding a letter usually works
On a computer you will need to install a language keyboard which allows you to use the accent