12"Proceedings: Fourth Excursion: Magh Adhair." Journal of the Royal Society of Antiquaries of Ireland 10 (1900): 440.
More details may be found here. According to Professor Duffy, the Flan Sunagh chessboard story is likely a later romantic invention with no basis in fact. (Duffy, Seán. "Beal Boru and Magh Adhair on Voices from the Dawn." Message to the author. 1 May 2014. E-mail.)