McGuinness, is an Irish surname, meaning the "son of Angus", which in eastern Ulster was commonly pronounced in Irish as Mac Aonghusa. A prominent branch of the Uíbh Eachach Cobha, the Magennises would become chiefs of the territory of Iveagh, which by the 16th century comprised over half of modern County Down, Northern Ireland. By the end of the 17th century, their territory had been divided up between them, the McCartan chiefs and British prospectors.
The four main branches of the Magennis clan were: Castlewellan, Corgary, Kilwarlin, and Rathfriland, between whom there was some rivalry. The Mac Artáin McCartan chiefs of Kinelarty, descend from the same genealogical line as the Magennis clan, through Artán, the son of Faghártagh, the son of Mongán Mac Aonghusa.