It's generally a bad idea to change them at all. If she works in Ireland regularly and get's paid in Euro's why not get a Euro account and use that money when in the Euro zone.
She needs to transfer some money into her Graduate account in UK but although she has an Irish Bank Account the charges for transferring money to a Uk bank and from Euros to Sterling are horrendous. She also gets charged for all transactions she makes so is tending to keep hold of the cash.