The following year, 1938, at the age of 18, Cahill joined the local Clonard-based 'C' Company of the Belfast Brigade of the Irish Republican Army. By 1942, Cahill was serving as second in command. That year, during an anniversary march by the IRA for the Easter Rising, Cahill got into a shootout with five other IRA men against four Royal Ulster Constabulary Officers. Several men were wounded and Constable Patrick Murphy was killed. Cahill and four of the other men spent time in prison in Belfast, where they were interrogated daily. Tom Williams, the sixth IRA man to be charged, spent time in the Royal Victorian Hospital due to his injuries. It was there that he made a statement taking full responsibility for killing Constable Patrick Murphy. All six men were found guilty and sentenced to death in August 1942. The men's legal team managed to suspend the execution date after the verdict. An appeal campaign began and 207,000 signatures were collected. The United States State Department and the Vatican also supported the campaign. As a result, the men's sentences were changed to life in prison, except for Tom Williams who was executed.