Taoiseach Enda Kenny was heckled by about 100 protesters as he arrived at a ceremony honouring the Irish who fought and died in the US civil war.
The anti-water and anti-war protesters had said they would turn their backs in silence during the event in Ballymote, Co Sligo.
However, several demonstrators drowned out the Taoiseach and continued to shout as the ceremony got under way.
Mr Kenny and US Ambassador Kevin O'Malley jointly unveiled a bronze sculpture of a soldier on horseback at the event.
Protesters shouted “traitors”, “war mongers” and “American troops out of Shannon” throughout the Ambassador’s speech.
The monument marks the 150th anniversary of the ending of the Civil War and is dedicated to the memory of the Irish who served and died during the battle.