While at an ATM many of the machines will ask if you want to be charged in your home currency OR the local currency (€ 0r £).
We suggest choosing the local currency (if available) with this option your bank/credit card company will pre-determine the exchange rate (which may be more favourable for you, especially if they have no International Transaction Fees associated with your account).
If you choose your home currency then the establishment you are in will determine your exchange rate (note: you may also still be charged an International Transaction Fee even if it is your home currency, this will be dependent on the type of ATM and the provider e.g Bank of Ireland).