I think someone suggested here that "cad" as in "cad atá" is artificial standard Irish, and that natural Irish would use "cad é" or "céard" instead. For your information, "cad" is used in precisely this way in Cairbre Irish (Cairbre, i.e. Ros Ó gCairbre/Rosscarbery, Co. Cork).

Reference: "Cad tá air" on Page 451 in: Seanachas ó Chairbre 1. Seán Ó Cróinín a thóg síos, Donncha Ó Cróinín a chuir in eagar. Comhairle Bhéaloideas Éireann, Baile Átha Cliath 1985.

The book is in very natural spoken Irish, and very unedited, so it is a perfectly reliable source.

That was David Webb, I believe. Here is the previous thread:

http://www.daltai.com/discus/messages/20/50379.html?1272633675