I'm trying to understand to logic why "é" is used here. I'm trying hard, my brain wants me to stop. :(
I totally understand it if it's a statement: "It is your question" --> "Is é do cheist." But when it's a question I'm not so sure. The copula is implied in "Cad", OK, but still it's hard for me to understand what the "é" is doing here. Does the sentence literally mean "What is (it) that IT is your question?"? If the sentence literally does mean this, then, is "é" the "that it is" part? If this is correct, to me it looks like there should be a second copula in the sentence for it to make sense.
So complicated... :( sorry again for the rambling.
bhfuil is not "the standard copula". bhfuil is the eclipsed form of the dependent present tense of the verb bí. This is not considered a copula in Irish.
The answer to "what is" is a copular statement, as you use a noun or pronoun to classify or describe a noun or pronoun. The answer to "where is" is not a copular statement. When the answer is copular, the question must be copular too, (either explicitly or implicitly).