This family name originated in Iveagh, in what is now Co. Down. From the 12th to the 17th centuries, the seat of power was the town of Rathfriland in Co. Down. Here the Magennises (another spelling of MacGuinness) had their main castle, which was destroyed in 1641. The name comes from the Irish, meaning son of Aonghus (aongus means one choice), and can be traced back to the 5th century and the time of St. Patrick: the sept was descended from Saran, Chief of Dal Araidhe (Dalriada), an ancient territory of the North.

The name often has Northern connections. The Guinness family of brewing fame was originally from Co. Down. Richard Guinis was the agent to the Protestant Archbishop of Cashel in Co. Tipperary. Hop plants growing in the grounds of the Archbishop's Palace were used by Guinis to produce the dark coloured beer that his son Arthur brewed when he started the world-famous St. James's Gate brewery in Dublin in 1759.