The second son of The 3rd Baron Rossmore and Josephine Julia Helen (née Lloyd), he was educated at Rugby.
His brother had been an officer of the 1st Life Guards, and Derrick was transferred to that regiment to replace him, as a sub-lieutenant, on 10 June 1874.
[4] Lord Rossmore was appointed Honorary Colonel of the 5th Battalion, Royal Irish Fusiliers (formerly the Monaghan Militia), on 29 July 1896.
On 18 June 1897, he was appointed Lord Lieutenant of Monaghan, succeeding the late Earl of Dartrey, and held the office until his death.
He married Mittie Naylor on 14 June 1882 at All Saints' Church, Ennismore Gardens, London, England.