He attended The Abbey School, Tipperary and graduated from Trinity College Dublin.
[5] He married his first cousin once removed, Adela Elizabeth Richards, on 18 August 1880.
[6] Orpen's main work was Ireland under the Normans, a four-volume work of a total of c. 1500 pages, first published by Clarendon Press 1911–20, and then reissued in 1968.
Ireland under the Normans generated political controversy when it was published, as Orpen "affronted many fellow Irishmen with his contrast between Ireland’s ‘progress, vigour and comparative order’ under Anglo-Norman rule, and ‘retrogression, stagnation, and comparative anarchy’ under ‘the recrudescence of Celtic tribalism’ in the two centuries after 1333".
Orpen died a widower at Monksgrange House, Grange Demesne, County Wexford, on 15 May 1932, aged 80.