Note the royal cipher G R.George Rex (King George in Latin) with the crown between the G and R. This signifies that it was made during the reign of King George V 1910 -1936. You can read the words Post Office on the aperture.
It would originally have been red and made of cast iron but with the formation of the Free State they were painted green. These King George V postboxes are rare in Ireland.
Kilbeacanty, County Galway