When Ireland was under British rule the postboxes were as on the British mainland: coloured red. When Ireland got independence in 1922 those postboxes were retained but were coloured green to identify them, I suppose, as non-British. There are still some to be found around the country as in this example in County Waterford. It dates from the reign of King George (1910-1936) hence the "GR" mark. It is still being used as a postbox.
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