"What is her address?"
Translation:Cad é a seoladh?
"Cén seoladh atá aici?" is a a perfectly acceptable way to ask "What is her address?" and should be accepted here. If anything, "Cad é a seoladh?" sounds more like you're probing the idea of "What concept underlies her address?"
What's the difference between using "cad é" and using " cad atá"?
Interrogatives such as cad include an implicit copula. A more literal translation of this sentence would be “What is it, her address?”, with the implied copula corresponding to “is”. An atá isn’t needed for a simple question like this sentence.
I know I will sound dumb for asking this, but why is seoladh not lenited in this case?
It's not dumb at all! Nouns are lenited after 'a' when it means 'his', not 'her':
His address: a sheoladh
Her address: a seoladh
Their address: a seoladh (if the noun started with b, c, d, f, g, p or t it'd be eclipsed after 'their')
For more, see https://www.duolingo.com/comment/10995361