Callahan family search

I am looking for my ancestors of the Callahan family. My great, great grandfathers name was M. W. Callahan. His son(my greatgrand father) was Richard Harold Callahan. His son(my grandfather) was Thomas Maston Callahan. I do not know who my great, great grandfathers father was. I know that W. M. Callahan was here in Piedmont,SC, USA, Anderson County because I found a World War One draft registration card for my great grand father, Richard, which he signed and dated on June 5, 1917  Richard was born on November 18th, 1890. Richard listed his father as M. W. Callahan as his employer and the family owned a farm. I am stuck at this point. I can't find anymore information on my M. W. Callahan so I have no idea who his father was. The family may have left Ireland in the late 1600's. I know this from a genealogist that I contact but I have not retained his services yet and this is not confirmed. I don't know why they left or where they went too when they left. Any help would be greatly appreciated