Employed by Arthur Guinness & Sons Brewery, St. James's Gate, Dublin O'Hannigan resigned his position. Some three weeks prior to the Rising he was sent by Irish Volunteers General Headquarters to Dundalk, County Louth to train and prepare Volunteers. O'Hannigan evaded capture following the surrender and escaped to the United States of America. In 1917 and 1918 O'Hannigan claims that he was involved in attempts to organise an arms landing from a German submarine. He further claims that from 1917 and throughout the War of Independence he continued his involvement in the smuggling of arms and personnel between Ireland, England and America including assisting in the movement of Eamon de Valera from England to Ireland following his escape from Lincoln Prison and the passage of Harry Boland from Ireland to the United States of America.