I am not sure I understand the question, but:
Quotation from link:
supplementary queries with Cá = "where is" This is also a copular construction, so the continuation of the question is only possible as:
•an indir. relative clause.
■with -verbs (cá fuses with a / ar to become the verbal particle cá / cár e.g.: Cá bhfuil tú = Where are you? Cár chuir tú é = Where have you put it ?) or
■with -copula (see below)
when as the relative clause with the copula : Combinations of cá and following the indir. relative form of the copula : cár (= cá + ar, cárb preceding a vowel)
Personal comment: "atá" seems to be possible in a lot of questions, but apparently not easily in where style questions?
http://nualeargais.ie/gnag/gram.htm / "other words" / "interrogatives"