“GB used in Ireland” is a phrase familiar to people who collect Irish Stamps. Britain has been issuing stamps from 1840. Thus during the period 1840 to 1922 (Irish Independence), stamps featuring Queen Victoria, King Edward VII and King George V were readily available in Ireland. At that stage Ireland was part of the “United Kingdom”.
Of course there is no way of telling if a Queen Victoria stamp was used in Ireland unless of course it is preserved on a cover or postcard and the date and postmark are clearly shown.
Most Dealers have boxes of postcards from the “golden age” of postcards, when they were used to send simple messages in the days before the Telephone took over. Edwardian Postcards are not rare. What makes them interesting to Postcard Collectors is the “picture” or subject (and possibly the publisher is collectible) but what interests the Stamp Collector is the stamp and the postmark.
After Independence, Irish postmarks were in the “first official language” (Irish or Gaelic). For example, the postmark “Cork” became “Corcaigh” and “Sligo” became “Sligeach” so although not especially rare…these items are sought after by Stamp Collectors and Postcard Collectors.
It is also worth bearing in mind that after Independence, the names of some towns were changed as they were clearly inappropriate. Thus postmarks of “Queenstown” (it was re-named Cobh) and “Maryborough” (Port Laoise) are sought after. Likewise an envelope addressed to “Queen’s County” (now County Laois) and King’s County (County Offaly) have a philatelic interest as well as general historic interest.
Of course British stamps are still used in Ireland…in the six counties in the North East, designated “Northern Ireland” and still officially part of the “United Kingdom”. It would of course be impracticable to collect every single stamp with a postmark in the Six Counties.