Members of the famed Guinness brewing family had their DNA tested to confirm their ancestor Arthur Guinness's claim that he descended from the Magennis chieftains of Iveagh, in County Down. It turns out that he descended from "the subsidiary McCartan clan, a far less eminent family."
The book [Arthur's Round: The Life And Times Of Brewing Legend Arthur Guinness] explains that where Arthur's genuine ancestors, the McCartans, once lived is a small village called Guiness or Ginnies.
The name of which is derived from the Irish Gion Ais, meaning wedge-shaped ridge -- thus clarifying the roots of the famous surname.
However, the pretensions arose when Arthur Guinness married in 1761 and engraved a silver cup with the armorial bearings of the Magennises of Iveagh -- a lion, with the red hand of Ulster, and a boar. [Link]