Airmail Covers. 
Flown airmail first flight covers are quite collectible. In the early days of aviation, a new flight route was remarkable. While Ireland did not have any airports, as such, until after WWII, planes of that era only required a fairly level grass strip and a wind sock. Some of the earliest airmails were used to expedite trans-Atlantic mail. The Germans catapulted airplanes from ships with mail to arrive a day or two before the ship. Onward flights were to London and Hamburg from Galway. (Figure 1).
The British Great Western Railway established an airmail system which had Belfast as a terminal during the mid-1930’s. Charles Lindbergh flew over Dingle and Valentia Island to get directions to Paris, but didn’t stop. (Figure 2)
Trans-Atlantic flights, however, did not reach reality until the arrival of the Pan Am Boeing B-214 Yankee Clipper seaplane in June 1939. (Figure 3) This flight stopped at Foynes, located in the Shannon estuary near Limerick for fuel. It continued on to Southampton to connect to the ocean liners. With the start of WWII on 3 September, the neutral American flag airplanes could not continue on to England since that placed them in a war zone. The Pan Am route terminated at Foynes and mail and passengers transferred to Imperial Airways seaplanes for the remainder of the flight to other locations in England.
During WWII, the Pan Am route continued bringing vital mail and VIP passengers to Foynes. All mail, however, continued directly to England for censorship, and that mail going to Ireland was returned later. A reverse procedure had Irish mail going to England first. The cover in Figure 4 from Ballinarobe, Co. Mayo to Wilmington, DE has the pink Irish censor tape, and the British PC90 from censor 5743 in Liverpool, and was returned to Foyes to meet the Clipper.
After WWII, the Boeing seaplanes were retired and replaced with land-based aircraft of greater speed and capacity. A commercial large airport was built to the northeast of Foynes, Shannon, along with airports at Dublin and Belfast. With the advent of jet aircraft, the location near the Atlantic was no longer critical. (Figure 5). Today, most traffic for Britain goes to Gatwick or Heathrow, while Ireland-bound passengers go to Dublin. Shannon remains busy however, with a large amount of airfreight. The shore facilities at Foynes have become a museum (Figure 6).
Collecting flight covers is an interesting part of aviation and postal history.
Irish Airmail 1919-1980, Bill Murphy, Eire Philatelic Assn.
American Air Mail Catalogue, American Airmail Society