Between 1942 and the end of the Second World War, around 300,000 American service personnel passed through Ulster. At its peak, U.S. military personnel made up around 1/10th of Northern Ireland's population in some areas.
American troops were scattered across the country as they prepared to enter battle in North Africa and across Europe. While training, they opened Irish eyes to American culture including 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 U.S.A. during peacetime.
34th Infantry Division land in Ulster
On 26th January 1942, Lieutenant J.R. Bainbridge took a series of photos as the first elements of 34th Infantry Division, United States Army arrived at Dufferin Dock, Belfast.
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.