Cahill was the first child of eleven siblings born to Joseph and Josephine Cahill. Both of his parents supported republicanism. His father was involved with the Irish National Volunteers and would print republican material at his print shop. Joseph Senior applied to be a part of the Irish Republican Army but was asked to remain in the print business as his way of assisting the republican movement. He was arrested in 1932 for printing illegal material, but was acquitted for any crimes. Cahill's childhood was marked by hardship and his family was very poor. Cahill's grandparents were neighbours of the Scottish-born Irish socialist and Easter Rising leader James Connolly, who co-founded the Irish Citizens Army.