Just watch that the advance payment, when the transaction is 'pending', will be different to the amount eventually debited. The actual debit will always, IME, be a few cent dearer.
I would assume that the higher difference in price than expected would be due to the fact that there will be a currency conversion involved in paying the Irish tax - it's not as straightforward as saying that the exact amount charged should be purchase price divided by 1.2 and then multiplied by 1.23, as it then has to be converted to Euro to actually pay the tax, which will incur additional expense for the retailer.
As previous posters have pointed out retailers tend to charge more if they are doing the conversion than if it is being done by the card provider so I don't see why it would be any different here.