Sometimes a customer may be charged more than the order total if your restaurant is based outside of Ireland. It is not a foreign exchange fee, rather the customer's bank is charging them extra to send a payment to a merchant who is outside your region.
What should happen, and what is happening with most of your customers, is that you are presenting the charge in your currency and it is being ‘settled’ in your currency. Creating charges this way allows you to absorb the foreign exchange fee instead of your customers.
Unfortunately, the customer may also be charged a fee by their credit card company if the credit card and your business are in different countries, regardless of how you present the currency.
Each individual bank makes the determination of what to show on a customer's statement (and whether or not they charge foreign transaction fees) and isn't something Flipdish have control over.
I'd encourage you to have your customers to reach out to their respective banks for more information, and to have them encourage their banks to list these charges more clearly on the cardholder statements.