‘At that time the postage stamps used in Ireland were the same as those in use in England showing the head of King George V. Patriots made a point of always stamping their letters with the King’s head upside down.’ 
I came across this quote during the summer, from a description of the period between the 1916 Rising and the War of Independence kicking off ca. 1918-19.
It’s from an unpublished account by Euds Tonson-Rye who was at the time, a boy of 8 or 9, growing up in the ‘big house’ at Ryecourt, near Cloughduv and Crookstown, Co Cork. It stood on the Ryecourt estate, just south of the Cork Macroom Road, between Farran and Lissarda.
I’ve just, coincidentally, acquired these stamps, cancelled in Macroom. They’re from the slightly later post-Truce period in February 1922, the heady period before the Civil War when, before the British stamps were removed from use, they were imprinted ‘Rialtas Sealadach na hEireann’ [Provisional Government of Ireland]
You may see the head of King George V a little more clearer in this other one below, which I’ve turned ‘the right way round’ for ease of vision.
The third photo, below, suggests the sender was one of the many in the Macroom district who, in 1923, was not pleased either to be under the government of the Irish Free State as they, too, look to have placed their stamp upside down on the envelope.
 – Euds Tonson-Rye, typescript history of Ryecourt, Co Cork (ca. 1972), U 271, Special Collections, Boole Library, University College Cork.