Brian Boruma mac Cenetig is my 39th GGrandfather. His son, Donchadda mac Briain, by Gormflaith ingen Murchadha, is the father of Pluinceid mac Donchadda, father of Oliver of Bolin, father of Walter Plunket, the first man to use the Plunkett surname. I am descended from the Plunket's of Rathregan, Dunsany, and Rathmore.
Found out Brian Boru is my grandfather a few years back. We did our family history and traced back to him he is as far as we got. Gets a little harder with all the different last names back in that time. They changed their names so often its hard to find who was his ancestors. I read the book Lion of Ireland. Very good read, but I believe some of that was probably fictional compared to other stories of read about him.