Between 1942 and the end of World War Two, around 300,000 American service personnel passed through Ulster. At its peak, US military personnel made up around 1/10th of Northern Ireland's population.
American troops were scattered across the country as they prepared to enter battle in North Africa and Normandy. While training, they opened Irish eyes to American culture. This included sports such as American Football and Baseball. By the end of the Second World War, American GIs had made a huge impression on the people of Northern Ireland. In fact, almost 2,000 women from Ulster became GI brides and left Irish shores to start a new life in the USA during peacetime.
Without Northern Ireland, I do not see how the American forces could have been concentrated to begin the invasion. The city of Belfast, its facilities, people, and influences made possible the beginning of that concentration.
General Dwight D Eisenhower, Speech at Belfast City Hall, August 1945.