At some point in the mid-1950s, the IRA approached Cotter and Harrison about supplying them arms for their Border Campaign, a request which the pair accepted, beginning a gun running career which would last for several decades.

Harrison was able to source arms for the IRA through George de Meo, an Italian-American neighbour of his who had connections to the Mafia as well as gun runners who were supplying rebels in Cuba.

[2] By 1975, the United States Department of Justice started to significantly weaken the Harrison network by prosecuting dozens of IRA arms trafficking cases since the start of the Troubles in 1969, and as early as 1973, the IRA already found more lucrative sources of funding and weapons from foreign states, including Libyan dictator Muammar Gaddafi.

"[5] U.S. House Speaker Tip O'Neill told Northern Ireland Secretary of State Roy Mason in mid-October 1977 that "[t]he flow of guns and money had been greatly reduced.

[1][7] Harrison, Tom Falvey, Michael Flannery, Pat Mullin, and Danny Gormley, all Irish immigrants, were acquitted in a criminal trial held in New York City, which lasted from December 1980 to June 1981.

Their acquittal was widely attributed to the unconventional efforts of Harrison's personal attorney, Frank Durkan; the men admitted to their activities but claimed that they believed the operation had Central Intelligence Agency sanction.