His son Robert seems to be married to his mother. Robert's will still survives, an abstract of it is:
"TRIMLESTOWN, LORD, ROBERT BARNEWALL called Lord Trimlestown. 4 Dec. 1779. Precis | p. 14 Dec. 1779. His wife Anna Maria Barnewall called Lady Trimlestown exor. To his son Joseph Barnewall exor. all his estate of inheritance and all other estate in manner or reported manor of Ro[e]buck, and Glanskeagh otherwise Clonskeagh, Co. Dublin, subject to charges thereon made by settlement on his marriage with his then present wife. His daughter Alicia Barnewall. Witnesses: Tyrrell O'Reilly, Dublin, Esq., Elizabeth Harris, Primrose Hill, Co. Dublin, spinster, Thomas Dennis, gent., agent to testator. Memorial witnessed by: Tyrell O'Reilly, Christopher Abbott, junior, Dublin, Esq. 330, 242, 221928 Joseph Barnewall (seal) (The Hon. Joseph Barnewall)"
That was his third wife. He had married his second wife by 1747 as per a court case of that year "Plaintiffs: Robert [Barnewall] Lord Trimlestown in Ireland and Elizabeth [Barnewall] Lady Trimlestown his wife and Mary Colt, spinster, infant (by said Robert [Barnewall] Lord Trimlestown) (said Elizabeth [Barnewall] Lady Trlmlestown and Mary Colt are daughters of John Colt, citizen and draper of London, deceased, and sisters of John Colt, clerk, younger son of said John Colt, draper, late a plaintiff and now deceased). "