Fitzgerald joined the British Army and became a lieutenant in the 66th Foot in 1788, transferring as a captain in 1779 to the newly raised 85th Foot, which was posted to garrison duty in Jamaica for the duration of the American Revolutionary War. He was there promoted to major in 1781 and lieutenant-colonel in 1783, taking over command of the regiment from General Charles Stanhope, 3rd Earl of Harrington. After the 85th was disbanded in 1783 he became a captain and lieutenant-colonel in the 2nd Foot Guards in 1789 and retired from active service in 1792.[1]
He married Charlotte Boyle Walsingham in London on 3/4 August 1791. Together they lived at Boyle Farm, Thames Ditton. In 1806, his wife succeeded to the Baron de Ros, the premier baron in the Peerage of England, as 20th Baroness de Ros. The family then took the surname FitzGerald-de Ros.
The couple had 13 children in 21 years, of which nine survived to adulthood:
Hon. Edmund Emilius Boyle FitzGerald-de Ros (4 May 1799 – 12 September 1810), died in childhood
Hon. Charlotte FitzGerald-de Ros (1801–1813), died in childhood
Hon. Henrietta Mabel FitzGerald-de Ros (1802 – 22 December 1879), married on 24 October 1828 John Broadhurst of Foston Hall (d. 1861)
RAdm. Hon. John Frederick FitzGerald-de Ros (6 March 1804 – 19 June 1861)
Augustus FitzGerald-de Ros (b. 1805), died in childhood
Hon. Olivia Cecilia FitzGerald-de Ros (11 January 1807 – 21 April 1885), married on 22/23 October 1833 Henry Wellesley, 1st Earl Cowley
Hon. Geraldine FitzGerald-de Ros (1809 – 28 September 1881), married on 25 November 1830 The Rev. Frederic Pare
Hon. Cecilia FitzGerald-de Ros (1811 – 6 October 1869), married on 10 December 1835 The Hon. John Boyle (1803–1874), son of Edmund Boyle, 8th Earl of Cork and had issue
Hon. Jane FitzGerald-de Ros (1813–1885), married Christopher Hamilton, and had issue[citation needed]