Postmarks - as distinct from the stamps themselves - are collectibles in their own right and can be worth hundreds of pounds.

According to Mr Ian Whyte of Whyte's auctioneers in Marlborough Street, Dublin, the earliest postal marking in Ireland was in 1670. Called a bishop mark, they were invented by a postmaster named Bishop. Postmarks began in England in 1660.

"All they had would be the number, so a marking struck on October 29th would have had a small circular mark in two semicircles - a circle with a split down the middle of it." On its top would be 29, with Oc below for October.

"They didn't give a year as such. You'd only know the year by the date the person had written on the letter," explains Mr Whyte.

This was before envelopes and the letter comprised a sheet of paper, folded into three with a wax seal on its back.

"So most of the time you get the complete letter, so you can actually tell what year it is," he says.

The earliest postal markings are the most valuable. An Irish marking from 1670 could fetch £2,000 (€2,540).

"Go a year or two later, and you're down to £500 to £1,000. And after 1700 they drop dramatically down to £20 or £30," he says.

The earliest markings were quite small, about 14mm in diameter, so they're hard to find. "And the other thing is to get them well struck," says Mr Whyte.

As for the likelihood of finding such early postal markings, there are archives in Ireland with letters going back that far. "People haven't looked to see if they've got postal markings on them," he says.

"Sometimes archivists have asked us to go through their archives to see if there is any philatelic material in them - material of postmark or stamp interest. Then they'll have a look at that and see is it important to the archive. If it's not, they'll sell it and use the money to buy something that is important to the archive. Because they're not keeping a stamp collection, they're keeping an archive of documents that related to their particular discipline or activity."

Town stamps are another type of postmark, from about 1720. Generally a straight line marking - Dublin, Waterford, Cork and Limerick were among the early ones. "And you get interesting spellings, for instance you'd find Limrick for Limerick." Castlebar is rendered Cl.bar.

Town stamps range in value from £30 to £600. Waterford - all in capitals but with a larger W - and the Strabane town stamp from about 1699 could fetch up to £600, he says.

Mileage stamps began around 1810, when people were charged by distance. They added to the town stamp how far it was to Dublin. They are not very rare and tend to be worth from £10 up to £60.

But a "fancy looking" one such as Gorey 48 or Lurgangreen 37 (near Dundalk) can fetch up to £90, he says.

Penny posts were also set up within towns, whereby the charge would be a penny to deliver a letter within a town. "So you then get the name of the town with the word penny post after it or underneath it," says Mr Whyte.

Some penny posts can be valuable. "We had one called Cross Poddle, as in the Poddle river, with penny post written underneath. One of the rarer Dublin penny posts, it sold for about £250. But you can pick up some for as little as £15 or £20 - so there's quite a range."

Stampa 99, the 28th national stamp exhibition, takes place from Friday, November 12th, to Sunday, November 14th, at the RDS, Ballsbridge, Dublin.

Joe Armstrong is at jmarms@irish-times.ie