I'm usually very careful to make sure my card is charged in euros when in Ireland, not US dollars.
However, I was buying 100 euro gift certificates from two different restaurants, and when checking out at the first one, they said "Charge it in dollars, yeah?", and I just said yes without thinking.
At the second restaurant (a few minutes later), I made sure they charged in euros.
The difference? An extra $5.09 conversion charge for the US dollar mistake (see pic).
Nothing to worry about, but that's why I always tell people to make sure they don't allow charges in US dollars. They make it sound like they're doing you a favor, but in reality they're charging a hefty exchange tax. Of course, you should always use a card that doesn't charge foreign conversion fees.