In 1984 he notified the Garda of an attempt to smuggle seven tons of AK-47 assault rifles from the United States to Ireland aboard a fishing trawler named "Valhalla." The guns were intended for the arsenal of the Provisional IRA's units. The shipment had been organised by the Winter Hill Gang, an Irish-American crime family of South Boston, Massachusetts. As a result of O'Callaghan's warning, a combined force of the Irish Navy and Gardaí intercepted the boat that received the weaponry, and the guns were seized.