 Fergus was a grandson of Enna Nia mac Bressal Bélach.One of Bressal's other grandsons was Énnae Cennsalach, from whom many Kings of Leinster descended, so the Uí Fergusa were like the Uí Ceinnselaig of the Laigin nation.Sources such as the Book of Leinster and Book of Ballymote state Uí Fergusa was situated between the river Liffey and Fir Cualan (Cuallan).