Setanta now you're just writing before your thinking. The IRA did not arise in America, General Michael Collins started the IRA and they did not get any backings from the Americians in their war in 1917.
The Fenian Brotherhood, founded in New York City by John O'Mahony in 1858, was a sister organization to the Irish Revolutionary Brotherhood organized in Dublin by James Stephens in the same year. Its aim was to rid Ireland of English rule by providing American money and manpower to encourage insurrection. By 1865 it had attracted 250,000 followers, many of them Civil War veterans, and an Irish Republican government on the American model had been set up at Philadelphia.
Fenianism, however, also spawned dissident factions hoping to benefit from Anglo-American conflict by attacking Canada. On April 12, 1866, an attempt to seize the Canadian island of Campobello in the Bay of Fundy was thwarted at Eastport, Maine. The British and American navies cooperated in intercepting a shipment of arms, and U.S. troops under Maj. Gen. George Meade forced the Fenians gathered at Eastport to disperse. On June 1, Col. John O'Neill defeated a Canadian militia company before retreating to Buffalo. O'Neill's second foray into Canada, on May 25, 1870, was easily repelled. Stephens, fleeing from the threat of arrest in Ireland, arrived in New York on May 10, 1866. Denouncing the attacks on Canada, he ousted O'Mahony from the leadership of the movement and reasserted the goal of insurrection on Irish soil alone. Betrayed by police informants, this goal ended in small sporadic rural uprisings on March 5-6, 1867. Fenianism collapsed in both countries, but Irish nationalism had taken on an irrevocable American dimension.
There are a few references to the Fenian troops who crossed the Niagara River near Buffalo calling themselves the "Irish Republican Army" including a painting of the battle in the National Archives of Canada showing a green flag with the letters IRA over a gold harp. This is considered to be the first ever use of the term.
News flash for RP: Ireland is not a part of the UK.