Cahill was educated at St. Mary's Christian Brothers' School, then located on Barrack Street. At age 14 he left school to assist in the print shop. Soon after, he joined the Catholic Young Men's Society, which campaigned on social issues with a focus on eradicating moneylenders from working-class areas of Belfast, as they often charged usurious interest rates. At the age of seventeen, Cahill then joined Na Fianna Eireann, a republican-orientated Scouting movement. Na Fianna Eireann was regarded as the "Junior Irish Republican Army".