Later that year, Cahill was a key figure in founding the Provisional Irish Republican Army. Angry at the failure of the IRA (led in Belfast by Billy McMillen) to defend Catholic areas during the communal rioting, Cahill and McKee stated in September 1969 that they would no longer be taking orders from the IRA leadership in Dublin, or from McMillen. In December 1969, they declared their allegiance to the Provisional IRA, which had split away from the leadership. This action took 9 out of the 13 units of the IRA in Belfast into the Provisional IRA. The remnants of the pre-split IRA became known as the Official IRA. The Provisional IRA was created and an executive council of 12 men was elected. Of the executive members, seven, one of whom was Joe Cahill. were elected to serve as the Provisional Army Council. Cahill also served as the second-in-command to Billy McKee, OC of the Belfast Battalion.