The regulation unfortunately doesn't specify exactly how the breakage is to be calculated, but it is quiet specific that the cost to the consumer should not exceed the financial loss to the bank. A bank must provide a written breakage quote to the consumer, and list all assumptions used. All assumptions must be reasonable, and justified.
What this boils down to is that the breakage cost can't be more than the difference between what the bank was able to borrow money for for the original term in the inter-bank markets, minus what they can get for depositing the money in the inter-bank markets for the remaining term, times the remaining term.
@RedOnion and others: Since the mainstream banks in Ireland seem to fund (most of) their mortgage lending from their customers' deposits, on which they pay very little interest, are they justified in using interbank rates when calculating break fees?