Early life and family
O'Brien was the son of Edward William O'Brien and Hon. Mary Spring Rice, granddaughter of Lord Monteagle of Brandon. He was born at Mount Trenchard House near Foynes in County Limerick. For a time after his mother's death, he was raised by his aunt Charlotte Grace O'Brien, along with his sisters, Nelly and Lucy. His father subsequently remarried in 1880. He was educated at Harrow School and Trinity College, Cambridge.
O'Brien married Mabel Emmeline Smyly, daughter of Sir Philip Crampton Smyly, on 8 March 1902. Together they had five children. His son Brendan, a surgeon in Dublin, married artist Kitty Wilmer O'Brien.
Unlike many of his Irish contemporaries, after graduating from Cambridge O'Brien did not study art in Dublin, opting instead to travel to Paris, where he studied the paintings at the Louvre. In 1887, O'Brien visited galleries in Italy and then enrolled at the Royal Academy in Antwerp. At the Academy he was a fellow student of Walter Osborne. O'Brien left Antwerp in 1891 and returned to Paris, where he studied at Académie Julian. He relocated to London in 1893 and then Dublin in 1901. O'Brien was designated an associate of the Royal Hibernian Academy in 1906, a member in 1907, and was later president between 1910 and 1945. He was made an honorary member of the Royal Academy, London in 1912.
- Hourican, Bridget (2009). "O'Brien, Dermod". In McGuire, James; Quinn, James. Dictionary of Irish Biography. Cambridge: Cambridge University Press.
-  Dermod O'Brien, Irish Art. Retrieved November 30, 2006.