"We're Irish"

When Cormac/Robert, asked his father what he was, his father said they they were not Christians, Jews, or Catholics, they were Irish. As the story progresses, we learn that Cormac becomes a Celt, and there is a detailed description of the "Otherworld". Do you think that this connection to Ireland, Celtic history, etc., is provided by the author in order to set up his premise, that Cormac lives for hundreds of years? Or are there other reasons for this explanation of Cormac's early years?