looking for Robert Darnell 1906 Ancestors

My grandfathers name was Robert Lester Darnell.Born 4/22/1906, in Hot Springs Arkansas to Jim Walley Darnell and Ella Lucas.He told us stories of Indian ancestory - his grandfather married an Indian Princess? Cherokee, I think.
He told my uncle that his people were Irish and had come to America on a boat.
I know very little but would like to know more if anyone knows of these people.
They did move to Oklahoma when he was a teen (I think)
Please contact me if you know of these folks.
Thank you,
Patricia