I don't know anything about Irish cards, but outside of North America the cards tend to have currency exchange fees. These fees wouldn't be charged if you opt in to DCC. North American issuers have foreign transaction fees, which means that the fee happens regardless of the settlement currency. If the currency exchange fee is 3% and the DCC amount is 3%, it might make sense in that corner case to accept DCC. At least in the US, FTFs weren't included in the rewards calculation, but presumably the total with DCC would be. However, in practice this rarely works out since the typical DCC markup is > 3%.