26 January 1942: The 1st US troops to arrive in Europe in WWII disembarked at Belfast docks on this day. The 1st soldier credited with coming ashore was  Private First Class Milburn H. Henke of 34th (Red Bull) Infantry Division under the command of Major-General Russell P. Hartle. Their arrival had been triggered by Adolf Hitler’s Declaration of War on the USA just weeks before in the aftermath of the Japanese attack on Pearl Harbour.

At the quayside, the Union Flag and the Star-Spangled Banner fluttered, flanking the drab docking shed. Nearby, the band of the Royal Ulster Rifles awaited the moment when they would play to welcome their new transatlantic comrades. After what seemed like a wait of hours in the cold January air, the band struck up the British National Anthem. As the strains of ‘God Save The King’ floated across the docks, the Duke of Abercorn (Governor of Northern Ireland) arrived. Before long, the grey hulks of tender vessels appeared on the horizon. On board, khaki-clad, steel-helmeted troops lined the decks, eager to make their way ashore.

https://wartimeni.com/article/us-army-arrives-northern-ireland/ulster-welcome-for-major-general-hartle/

Other than a brief announcement that troops has indeed arrived in the ETO [European Theatre of Operations] the US Administration was loath to divulge the exact detail of the numbers and composition of the Expeditionary Force & where they would be based so as to deny the Germans any information as to their role in future operations.

Around 6,000 troops were part of that inaugural wave, with thousands more arriving before they eventually departed for the invasion of North Africa that autumn.

Their arrival was the start of a huge influx of US servicemen into the North that had profound but brief effects on the local populace incl. some 1,800 ‘G.I. brides’ & the introduction of Black soldiers into the local mix of a society unused to other than white people in their midst. More waves were to follow and used Ulster as base for training and as one of the launch pads for the invasion of Normandy on D Day 1944.

‘The first American troopship anchored off Bangor in January 1942 and it is estimated that over 300,000 GIs passed through Northern Ireland on their way to the front line.

Three huge US Navy battleships - the Nevada, Texas and Arkansas - arrived in Belfast Lough in April 1944 and, with D-Day preparations gathering pace, the fleet grew as more ships arrived, ready to deploy. On 3 June 1944, the ships left Belfast Lough for Utah and Omaha beaches in Normandy.'

https://www.bbc.com/news/uk-northern-ireland-48528858

While generally welcomed not all were happy especially the Irish Taoiseach Eamon De Valera who objected to foreign troops on Irish soil, albeit in a part of Ireland that was outside the control of the Irish Free State. The Department of External Affairs sent a telegram to  the Irish Legations at Washington and the Holy See and to the High Commission in London setting out the Nationalist perspective on the arrival of the US Army on Irish soil.

 

'In reply to the Press, Mr. de Valera stated today that the Irish Government had not been consulted either by the British Government or the American Government with regard to the coming of the American troops to the six counties.

Everyone knew, he said, that Ireland had twenty years ago been partitioned and the six counties cut off from the rest of the country by an Act of the British Parliament despite the expressed will of the Irish people....

The people of Ireland have no feeling of hostility towards and no desire to be brought in any way into conflict with the United States. For reasons which I referred to a few weeks ago, the contrary is the truth, but it is our duty to make it clearly understood that, no matter what troops occupy the six counties, the Irish people's claim for the union of the whole of the national territory and for supreme jurisdiction over it will remain unabated...

But the maintenance of the partition of Ireland is as indefensible as aggressions against small nations elsewhere which it is the avowed purpose of Great Britain and the United States in this war to bring to an end.' 

https://www.difp.ie/volume-7/1942/telegram-from-external-affairs-to-washington-dc-and-holy-see-legations-and-high-commission-london/3596/#section-documentpage

However De Valera’s objections were dismissed by Roosevelt as irrelevant and had no effect on the conduct of the US War effort whatsoever.