"What is his address?"
Translation:Céard é a sheoladh?
Cad, and the other interrogative pronouns, are forms of the copula (or are treated as containing a hidden copula). So you don't need an additional verb to say "what is this?" - cad é seo? - the "is" is part of cad.
When you add on an additional verb, like cad atá, it's more like "what is it that is" - cad atá ar an teilifís? can be understood as "what is it that is on the television?", but we usually just render this as "What is", but it's not the same "what is?" as cad é?
Which people say cad atá a sheoladh? all the time?
Scilling provides a succinct explanation for when to use cad é vs cad atá in this discussion:
If an answer to the question would use the copula, then Cad é … ? is used; if an answer wouldn’t use the copula, then Cad (é) atá … ? is used.
The answer to Cad atá a sheoladh? would be tá a sheoladh 1 Sráid Uí Chonnaill, rather than Is é 1 Sráid Uí Chonaill a sheoladh (or Is é a sheoladh 1 Sráid Uí Chonaill.