Why doesn't it work here to say "What is on television?"
Does this question not refer to TV programming? If it does, then "on television" should be accepted. It is only mandatory to say "on the television" if you are asking about something that is physically lying on the TV set.
Think of how you would reply to the question. If you would use tá, then ask the question with cad atá. If you would use the copula, use cad é.
- Cad atá ar an teilifís? Tá an nuacht ar an teilifís. What is on the television? The news is on the television.
- Cad é an cainéal? Is é CNN an cainéal. What is the channel? CNN is the channel.
- Cé hé an láithreoir? Is é James Earl Jones an láithreoir. Who is the presenter? James Earl Jones is the presenter.
Is there a rule of any kind for when you should use 'cad (é)' by itself and 'cad (é) atá'? For example, why is it 'cad (é) d'uimhir' and not 'cad (é) atá d'uimhir'? Does it have something to do with a definite word coming after 'cad (é)'? I've just been going off of feel so far, but I'd like a concrete rule to follow.