Complex sentence: Historians Peadar Livingstone & Robert Bell both claim the surname descends from Philip MacMahon (Irish name "Pilib MacMathghamhna") who was related to the Kings of Oriel.
Split into simpler sentences: Historians Peadar Livingstone & Robert Bell both claim the surname descends from Philip MacMahon (Irish name "Pilib MacMathghamhna"). He was related to the Kings of Oriel.