The price of the basic stamp increases from 48c to 55c tomorrow.
The rate for all international (airmail) letters up to 100g increases from 75c to 78c.
A 54c rate will apply to letters within Ireland prepaid by meter franking, Ceadunas licence or other business services.
The new rates cover postage on standard stamped mail up to 100g within the island of Ireland.
Currently, letters weighing 50g -100g cost 60c, a 5c saving for standard mail at this weight.
This is the first price increase for this category of mail since August 2003.
An Post has today defended the increase, claiming that rates are the fourth lowest in the EU15.
The new 55c stamp features a purple Large-Flowered butterwort, an insectivorous Wild Flower, common in areas of boggy ground in the south-west of the country.
The stamp illustration is by leading Irish botanical artist, Susan Sex.