Hi Marty
I am not sure what you mean. Fergus Mor is known as mac Eirc as he was the "son of" Erc. His father is not known as mac Earc. His father, if the legends hold any semblance of truth, was Erc mac Eochaid Muinremuir who, by the way, as I have noticed a couple of the father profiles was never King of Dal Riata. I think, if you heading back in time, it might be best to take this off line as we are definately into the myth and legend time.