The guys on the Northern Ireland board dug up this one
http://www.fifa.com/en/media/index/0,1369,74563,00.html
My reading of this is that if Alex Bruce's only qualification fdor us is through his grandmother from Co. Down then he would not be eligible. He may have a connection to the Republic as well but I haven't seen that clarified.
It would also rule out any NI born players from playing for us as the previous rule used to allow passport holders to play for a country. Anybody born on th eisland can get an Irish passport but now they will need a RoI grandparent at a minimum.