Born at Rossmore Park, County Monaghan, in Ireland on 14 November 1851, Henry Cairns Westenra, 4th Baron Rossmore, was the son of Henry Robert Westenra, 3rd Baron Rossmore (1792-1860) and Josephine Julia Helen Westenra. Baptised on 9 January 1852 at Monaghan Parish Church, he had two sisters and three brothers.
Henry succeeded to the family title as the 4th Baron Rossmore of Monaghan when he was only nine upon the death of his father on 1 December 1860.
He was educated between 1865 and 1868 at Eton College, Windsor. He then joined the 1st Life Guards, a cavalry regiment of the British Army.
At the age of 21 he took his seat in the House of Lords and also inherited large estates in County Monaghan and, from his father's first wife, Anne Douglas-Hamilton, daughter of the Duke of Hamilton and Brandon, a further estate in the Arran Islands.
Sadly, his time in the House of Lords and as owner of those estates was all too brief. He suffered a heavy fall when riding his horse Harlequin in the Guards’ Cup Steeplechase at Windsor on 20 March 1874.
Rushed to Windsor Hospital, he died there of a ruptured bladder on 28 March 1874, aged 22. He was buried in the family vault at Rossmore Park. He was unmarried at the time of his death.
His brother, Derrick Warner William Westenra (1853-1921), succeeded to the title as 5th Baron Rossmore and inherited the family’s estates following Lord Henry Rossmore’s untimely death.
Rossmore House was abandoned in the 1940s, fell into a ruin, and was eventually demolished in 1975.