Denis B. Cashman was born in Dungarvan in 1842. He was a member of the Irish Republican Brotherhood (Fenians) by 1858. One of his first activities may have been to help hide Lawrence O'Brien in his own home in the late 1850's. O'Brien had escaped from prison and was James Stephens and other leaders of the movement. He was arrested on 12 January 1867, was convicted Fenian membership and sentenced to seven years penal servitude in Freemantle, Australia. They sailed on board the hougoumont where Cashman compiled a diary of his voyage.

Cashman was pardoned in 1869 and went to San Francisco. He made his way to Boston where he was reunited with his wife and one of his children. He became involved in journalism, amongst other occupations. He wrote articles in support of the Nationalist cause, including a biography of Michael Davitt, after fleeing to America. The two met up in Boston 30 years later when they acted as pallbearers for the body of John Boyle O'Reilly.

In 1862 Cashman married his wife Catherine. They had three sons and two daughters. By this time he was working as a law clerk with the firm Dobbyn & Tandy in Waterford. He was arrested in 1867, by which time he had risen to the rank of Centre of the Waterford Circle, i.e. he had the power to enlist and swear in new members to the movement. By 1864 or 1865 Cashman was living at 5 Preston Street, Dublin. While in Dublin he met Dr. Edward Power, Davitt. Cashman died at his home in Boston in 1897.

A book has just been published by Wolfhound Press, entitled: Fenian Diary, Denis B. Cashman on board the Hougoumont 1867-1868, edited by C.W. Sullivan III. This includes poems by Cashman, John Boyle, John Flood and other Fenians written on board the transport ship.