Orpen was the son of Dr. John Herbert Orpen (1805–1888) and Ellen Susanna Gertude Richards (?–1855) and a second cousin of Sir William Orpen. He married his first cousin once removed, Adela Elizabeth Richards, on 18 August 1880.
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". A new one-volume edition was published by Four Courts Press in 2005.He also edited and translated The Song of Dermot and the Earl in 1892.
- Foster, Joseph (1885). . (second ed.). London: Hazell, Watson, and Viney. p. 346.
- . The Times. London. 19 May 1932. p. 1.
- Ashe Family - Dr. John Herbert ORPEN MD
- . The Times (46136). 18 May 1832. p. 12.
- Bull, Philip (2013). "Orpen, Adela Elizabeth (née Richards)". In McGuire, James; Quinn, James (eds.). Dictionary of Irish Biography. Cambridge: Cambridge University Press.
- Matthews, Elisabeth, Review: "Ireland under the Normans", University of Reading, retrieved 2010-03-19
- Seán Duffy, Historical revisit: Goddard Henry Orpen, "Ireland under the Normans, 1169-1333" (1911-20), Irish Historical Studies Vol. 32, No. 126 (Nov., 2000), pp. 246-259. JSTOR 30006999
- See also Donnchadh Ó Corráin. "Nationality and Kingship in Pre-Norman Ireland". CELT.
- Four Courts press Archived 2011-07-17 at the Wayback Machine
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Goddard Henry Orpen