Cahill is educated at St. Mary’s Christian Brothers’ Grammar School. At age 14 he leaves school to assist in his father’s print shop. Soon after, he joins the Catholic Young Men’s Society, which campaigns on social issues with a focus on eradicating moneylenders from working-class areas of Belfast, as they often charge usurious interest rates. At the age of seventeen, he joins Na Fianna Éireann, a republican-orientated Scouting movement. Na Fianna Eireann is regarded as the “Junior Irish Republican Army.”
Cahill joins the local Clonard-based ‘C’ Company of the Belfast Brigade of the Irish Republican Army in 1938. Four years later, during an anniversary march by the IRA for the Easter Rising, he gets into a shootout with five other IRA men against four Royal Ulster Constabulary (RUC) officers. Several men are wounded and Constable Patrick Murphy is killed. Cahill and four of the other men spend time in prison in Belfast. The IRA declares a formal ceasefire in 1945. Afterwards, republican prisoners begin to be released. Cahill is released in October 1949.
The IRA launches a new campaign in 1956. The IRA border campaign attacks ten targets in six counties, damaging bridges, courthouses and border roads. By 1957, three RUC officers and seven republicans have been killed during the campaign. Cahill is arrested and interned in January 1957 with several other republicans. He is released from internment in April 1961. Following his release from prison, he is disappointed at the direction of the IRA and resigns from the organisation around 1962.
In 1969, Cahill is a key figure in the founding of the Provisional Irish Republican Army. During his time in the Provisional IRA, he helps import weapons and raise financial support. He serves as the chief of staff in 1972, but is arrested the following year when a ship importing weapons was intercepted.
After his release, Cahill continues to serve on the IRA Army Council and leads all financial dealings for Sinn Féin. In the 1990s, the IRA and Sinn Féin begin to work on seeking peace. Cahill serves on the council that calls a cessation on July 21, 1996. He attends several of the talks that finally lead to the Good Friday Agreement on April 10, 1998. Shortly after the agreement is made, he resigns as treasurer of Sinn Féin. To honour his service, he is made honorary Sinn Féin Vice-President for life. He serves the Republican movement in Ireland all his life, as one of the longest-serving political activists in Ireland of any political party.
Cahill dies at age 84 in Belfast on July 23, 2004. He had been diagnosed with asbestosis, which he probably developed while working at the Harland & Wolff shipyards in his twenties. He and several other former shipyard workers later sue the company for their exposure to the dangerous substances but only win minimal compensation.