On 1 June 1875, Louisa married the 6th Earl of Antrim (thus becoming the Countess of Antrim) in the Chapel Royal at St.
James's.
They had three children:
Lady Sybil Mary McDonnell (26 March 1876 – 16 April 1959), married Vivian Smith, 1st Baron Bicester.
Randal McDonnell, 7th Earl of Antrim (10 December 1878 – 15 June 1932).
Hon.
Angus McDonnell (7 June 1881 – 22 April 1966).In 1890, the Countess became a Lady of the Bedchamber to Queen Victoria until the latter's death in 1901.
She was reappointed as such under Queen Alexandra in 1901 and until Edward VII's death in 1910.