1901 census has this Williamson family group at Ballee......http://www.census.nationalarchives.ie/pages/1901/Antrim/Kells/Ballee/930841/
Robert married Lizzie Graham, 23rd Sept 1880, Ballymena. Father named as William John Williamson. The witnesses were their fathers.
2nd son, William John, married Isabella Graham, 1917.https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1917/09733/5531546.pdf
Possible family of eldest son, James Williamson on 1911 census with wife Rebecca & 3 children. On there marriage, father listed as Robert.