G'day Ron,
(nice to see you back on SCF)
pre stam covers,
this is like the students getting a question from teacher

Just a few comments, not sure if it would be helpful.
First, there is a curious commentary on wiki
in the late 18th century there were only a few cabins and an Inn.
yet by the time your covers were being exchanged
there must have been a hive of activity
with over 5000 persons.
Being as the owner of Fermoy set up the mail coach system in
Ireland, one would suggest the mail costs would have been a given.
but the big query for me, would be why go via Dublin?
Surely County Cork would have had a major port
for receival of mails to the southern areas?
From wiki
In 1791, the lands around Fermoy were bought by a Scotsman, John Anderson. He was an entrepreneur who developed the roads and started the mail coach system in Ireland. He designed the town and the streets remain much the same as they were originally built. His descendants, now living in Australia, have named a wine after the town which he established.[6] A plaque and bust in his honour were unveiled by the town park in 2001. Fermoy was the site of a major British Army barracks, when Ireland was under imperial rule. By the 1830s this was the largest military establishment on the island of Ireland. In 1797, when the army was looking to establish a new and permanent base, Anderson gifted them the land as an inducement to locate in Fermoy. Anderson and the town received considerable economic benefit from the arrangement. In 1806 the first permanent barracks, the East Barracks, were built. They were located on 16˝ acres of land and provided accommodation for 112 officers and 1478 men of infantry, and 24 officers, 120 men, and 112 horses of cavalry. A general military hospital of 130 beds was also built. In 1809 West Barracks was built. This also had a 42 bed hospital. When both barracks were complete there was accommodation for 14 field officers, 169 officers, 2,816 men, and 152 horses.