Great Britain Used in Ireland, Part 2. 
It is not difficult to see English stamps used in Ireland. From medieval times to 3 December 1921, Ireland was a part of Great Britain. The Mulready envelope, invented by an Irishman, was early British postal stationery. (Figure 1). Fast forward to the Anglo-Irish Treaty of 1921 creating the Provisional Government of Ireland. Of course, the first stamps of the Provisional Government didn’t appear until 17 February 1922, and then they were British Stamps overprinted Rialtas Sealdac na h’Eireann for the Provisional government. (Figure 2) In the period 14 January 1922 (Ratification of the Treaty by the Dail) to 15 February, only non-overprinted British stamps were available, even though the Irish were theoretically in charge on 14 January. After February 1922, un-overprinted British stamps remained valid until 31 May 1922. Hi-value stamps (2/6, 5/ and 10/ remained as British “Seahorses” overprinted for the Provisional Government. (Figure 3). To complicate things, the Irish Civil War began on 23 June 1922, and the Irish Republican Army held some areas around Dublin and Cork disrupting mail movements. Of course, they had their own stamps (Figure 4) which were not recognized outside their territory.
On 3 November 1922, the 26 southern counties voted to form the Irish Free State. The new stamps were British stamps, now overprinted for the Free State of Ireland (Saorstát Eireann). (Figure 5). The full set was available by 16 December 1922. The first truly Irish stamp, a 2d map stamp, was issued on 6 December 1922 followed by other low values in 1923 but the complete set was’t available until 21 December 1923. (Figure 6).
The high values, 2/6, 5/ and 10/ remained as over-printed British “Seahorses” (Figure 7) until replaced by the Irish “Pascal Fire” stamps in 1937. (Figure 8).
British stamps continue to be used in Northern Ireland today. Specific regional stamps were issued for Northern Ireland starting in 1958 (Figure 9), followed by the ubiquitous Machins (Figure 10), and continue to the present Northern Ireland regional stamps (Figure 11).
Irelands Transition The Postal History of the Transitional Period 1922-1925, Doolin, Dr.C.I., MacDonnell-Whyte Ltd., Dublin, 1992.
The Complete Deegam Handbook, Myall, D.G.A., Bridport, Dorset, U.K., 1993.
Concise Great Britain Stamp Catalogue, Stanley Gibbons, Ltd, Ringwood, Hampshire, UK, various editions.