It's circa 1945 and was used, as noted, for the Irish postal savings system (the stamps were saved on a card and could be redeemed for a cash deposit). There are also some later pictorial savings stamps from the 1970s and perhaps other varieties.
I have seen these from time to time on
ebay, including on the form used to accumulate them for redemption, but I have not studied the Irish issues specifically, or tried to find any specialty literature on them.