Maritime Mail. 
Maritime mail is not limited just to large passenger-carrying ships; this can also include commercial cargo ships, ferry boats, and mail from visiting military foreign vessels.
Paquebot (the French term for packet boat) is that mail posted on a vessel on the high seas by passengers. This mail is conveyed to a specific port post office by a ship’s officer, normally the purser, and receives a special cancel. This is necessary since international rules allow franking with stamps of the country of vessel origin, stamps from the country of the previous port, or the stamps of the receiving country. This is normally seen on letters mailed aboard passenger cruise ships. (Figure 1). Military mail is usually not usually sent through civilian postal systems. The American Navy cancel, although from the Naval Operating Base in Londonderry is not paquebot mail since it was not mailed as being from a ship (although the writer may have been aboard), and it was completely mailed through American military postal channels and did not enter the English or Irish mail systems. (Figure 2).
Ferrys run regularly from Ireland to ports in Great Britain and France. While usually special cancels are available, the most significant amount is for special occasions, such as the first voyage of a new ship or for a new service or port entry. (Figure 3). Here an Irish stamp is legally used in Great Britain and cancelled by the Royal Post Office at Pembroke Dock.
Commercial vessels making an initial visit or a special service often have special cachets to mark the event. (Figure 4). The Greek Line’s ship’s hospital is different accomodation. The Dun Laoghaire cover does not identify the ship but cancel describes it was mailed at sea.. (Figure 5).
These above cancels follow similar rules as for the passenger ships; however, the ports used are often not accessible to passenger vessels. Over 70 Irish ports are known to have processed mail for shipping. Collecting an example from each port would be a significant challenge.