A half-length portrait to the right showing Casey in his lieutenant’s full dress uniform, which may be of the 1832–33 or, more likely, the 1833–43 pattern, but this cannot be ascertained because no cuffs are shown.
David O’Brien Casey entered the Navy in 1789 as a captain’s servant in the 24-gun ‘Hyaena’. On 27 May 1793, she was captured in the Caribbean by the French 40-gun frigate ‘Concorde’, becoming the first British loss of the French Revolutionary War. As a midshipman, he was in the ‘Hermione’ (32), the ‘Raisonnable’ (64) and the ‘Swiftsure’ (74), the latter two both flagships of Rear-Admiral William Parker. Early in 1797, after a spell as acting lieutenant in the ‘Ambuscade’, he rejoined the ‘Hermione’, which was now under the brutal command of Captain Hugh Pigot. In September 1797, Pigot noticed that one of the ties securing a sail had not been knotted: Casey as the supervising midshipman took responsibility for the oversight and apologized to the captain. Pigot demanded that Casey apologize on his knees but he refused to do so. Refusing a second time, Pigot ordered Casey to be flogged and disrated him. This could have effectively ended Casey's career and was entirely improper treatment of a midshipman. Following further unacceptable behaviour by the captain towards those working in the rigging, which resulted in three young men falling to their deaths, and a mass flogging of the topmen, mutiny erupted in the ‘Hermione’ on 21 September. Pigot, eight officers and two midshipmen were killed and the ship handed over to the Spanish, though retaken later in a daring cutting-out raid.

Casey, a popular midshipman and himself a victim of the captain’s wrath, was spared in the slaughter. He was promoted lieutenant on 11 October 1799 and served in the East and West Indies, the Mediterranean and with the Sea Fencibles in Ireland. On 6 December 1835 he was placed in charge of the semaphore station at Putney and then, on 15 October 1839, appointed a Lieutenant of Greenwich Hospital.