US Forces in Ireland – World War 1. 
World War I saw many Americans going “Over There”. But very few went to Ireland. No specific units were stationed in Ireland; however some American were sent there for “training”.
The largest contingent was maintenance troops of the U.S. Army Air Service. They were originally sent to England, and flying personnel were sent to various British airfields. The maintenance units were divided into flights of 40 to 60, and sent to various airfields to assist the British in aircraft maintenance. Four flights were sent to Ireland, and located at Baldonnel, Co. Dublin (now Casement AB); Tallaght, Co Dublin; Gormanston, Co. Meath; and Collinstown, Co. Westmeath with a headquarters in Dubllin. Figure 1 shows a cover from Tallaght, as seen in the cancel..
Mail for these troops was addressed to 35 Eaton Place, London SW1, and couired to the units. Outbound mail was sent via the British Post Office, and censored by British military censor. Later, American censors handled the mail. Only one or two pieces have been identified from Tallaght.
An American Army Port unit was assigned to Belfast. No mail is known. US Navy ships, mainly destroyers, operated from Queenstown (Cobh). A battlegroup operated from Bantry Bay. Six naval air stations also operated; Bereshaven, Castletown Bere; Lough Foyle; Wexford; Queenstown; and Whiddy Island. All mail was handled through Navy FPO channels.
The Military Postal History of Ireland, Kumpf, H-J., FAI, Publication 27, August 2008, Chapter 7.
“Fighter Training Camps”, Donohoe, B., Irish Philately, Sept. 2006, p.33-43.
“Irish Philately in World War I”, Blessington, J., The Revealer, Summer 1981, p.6-8.
“World War I Addenda”, Blessington, J. and “The AEF In Ireland”, Boyden, J.N., The Revealer, Winter 1981, p.47-48.