Mr Denis O'Brien 1 was born in Caheragh, Drimoleague 2, Co Cork, Ireland in early 1891.
He was the son of Michael O'Brien (b. 1851), a cobbler, and his wife Mary (b. 1850). As per the 1901 census, when he and his family were living in house 3, Caheragh, he had five siblings: Bridget (b. 1877), Kate (b. 1880), Margaret (b. 1882), Michael (b. 1884) and Mary Ellen (b. 1888). Denis later appears on the 1911 census living at house 20, Caheragh, the home of his married sister Kate O'Donovan and her family and he was described as a rural postman. His sister was listed as a "sub post-mistress."
Reportedly diminutive in stature, besides his postman duties Denis was also a gifted jockey, reportedly earning wins in his townland and further afield.
He boarded the Titanic at Queenstown as a third class passenger (ticket number 330979, which cost £7, 16s, 7d) and gave his local address as 196 Blarney Street, Cork. It is believed that his destination was the home of his sister Margaret in Manhattan. Family lore tells that his brother Michael, also resident in New York, sent him a coat in which to travel in so that he did not look too destitute travelling through Ellis Island.
Denis O'Brien died in the disaster. His body, if recovered, has never been identified.