Clancy is born at Grange, County Limerick in 1881 to a family with a strong republican tradition. He is educated at Crescent College, Limerick, and thereafter at the Catholic University in St. Stephen’s Green, now University College, Dublin. Among his friends at the university are James Joyce, Francis Sheehy-Skeffington, Tomás Mac Curtain and Terence MacSwiney. He helps form a branch of the Gaelic League at college and persuades his friends, including Joyce, to take lessons in the Irish language. He plays hurling and is a good friend of Michael Cusack. With Arthur Griffin he joins the Celtic Literary Society. It is said that he is the model for the character of Michael Davin in Joyce’s A Portrait of the Artist as a Young Man.
Clancy graduates in 1904 and finds a position teaching the Irish language at Clongowes Wood College and is active in the Gaelic Athletic Association. Due to ill health he has to return to his home at Grange. In 1908 he comes to Limerick to teach Irish. In 1913 he joins the Irish Volunteers. In 1915 he marries Máire Killeen, a teacher. After the 1916 Easter Rising he is arrested and imprisoned in Cork, but is released before he comes to trial following a hunger strike.
On the night of March 6, 1921 three Auxiliaries come to Clancy’s house and one of them shoots him, injuring him fatally. His wife is also injured in the attack. The previous Mayor, Michael O’Callaghan, is also murdered on the same night by the same group.
Suspicion immediately falls upon members of the Black and Tans, but a British inquiry into the murder, like most such inquiries through the years, absolve Crown forces of any blame. One of Clancy’s killers is later said to be George Nathan who dies in the Spanish Civil War in July 1937.