Martyn is the elder son of John Martyn of Tullira Castle, Ardrahan and Annie Mary Josephine (née Smyth) of Masonbrook, Loughrea, both of County Galway. He is educated at Belvedere College, Dublin, and Wimbledon College, London, both Jesuit schools, after which he enters Christ Church, Oxford in 1877, but leaves without taking a degree in 1879. His only sibling, John, dies in 1883.
Martyn begins writing fiction and plays in the 1880s. While his own output is undistinguished, he acquires a well-earned reputation as a noted connoisseur of music, both European classical and Irish traditional. He is a fine musician in his own right, giving memorable performances for guests on an organ he has installed at Tullira. He uses his wealth to benefit Irish culture.
Martyn is reportedly pivotal in introducing William Butler Yeats and Lady Gregory to each other in 1896. The three found the Irish Literary Theatre, for whom Martyn writes his best and most popular plays, The Heather Field and A Tale of a Town. He covers the costs of the company’s first three seasons, which proves crucial to establishing the company and the future of the Abbey Theatre. He later parts ways with Yeats and Gregory, something he later regrets, but remains on warm terms with Lady Gregory until the end of his life.
Martyn is a cousin and friend to George Moore (1852–1933). The two make frequent trips all over Europe, where Moore influences Martyn’s views on modern art, which result in the latter purchasing several works by Edgar Degas, Claude Monet, Jean-Baptiste-Camille Corot and Kitagawa Utamaro, all later donated to the National Gallery of Ireland. Moore does not share Martyn’s fenian ideas nor espousal of violent means to achieve national sovereignty. Their different political opinions eventually drive their friendship apart.
Martyn is descended from Richard Óge Martyn, a leading Irish Confederate, and Oliver Óge Martyn, a Jacobite who fights in the Williamite War in Ireland. Yet by his lifetime, the family are unionists. Martyn’s outlook begins to change in the 1880s after studying Irish history, as well as living through the events of the Irish Land War. He comes out as an Irish republican when he famously refuses to allow “God Save The Queen” to be sung after a dinner party at Tullira. By this stage he is involved with the political work of Maude Gonne and Arthur Griffith, and is a vocal opponent of the visit of Queen Victoria to Ireland in 1897. He also protests the visit by Edward VII in 1903, this time as chairman of the People’s Protection Committee. He is the first president of Sinn Féin from 1905 to 1908. In 1908 he resigns from the party and politics in general to concentrate on writing and his other activities.
He is on close personal terms with Thomas MacDonagh, Joseph Mary Plunkett and Patrick Pearse, and deeply mourns their executions in the aftermath of the Easter Rising. A parish hall and church that he founded at Labane, near Tullira, are burned by the Black and Tans. He supports the Anglo-Irish Treaty of 1921.
Martyn dies at Tullira on December 5, 1923 after years of ill health. Friends and family are shocked at a provision in his will that directs that his body be donated for the use of medical science and, after dissection, be buried in an unmarked pauper’s grave. The Palestrina Choir sings at his graveside. He bequeaths his papers to the Carmelites of Clarendon Street in Dublin, who subsequently misplace and lose them. Portraits of Martyn exist by, among others, John Butler Yeats and Sarah Purser. On his death the senior line of the Martyn family dies out. His property is inherited by his cousins, the Smyths of Masonbrook and Lord Hemphill. Tullira is sold by the latter forty years later changing ownership several times since.