Brian MacMahon

"[A]bout the middle of the fourteenth century we find the chief Brian, son of Hugh Mac Mahon, expelled from fortress of Rath-Tulad, now called Tully, a townland adjoining [to the south] the townland of Monaghan. This Brian, called The Great Mac Mahon, was treacherously murdered by one of his own people, in the year 1372."

Lots of work still to be done here.

• http://www.mcmahonsofmonaghan.org/nialls_pedigree_analysis.html