To be honest I'd say that's the least of your worries if the situation arises.
You might need to contact the Regulator on this one, but I imagine (I'm guessing though!) that it would just be the amount of money actually *in* the deposit account at the time, although it is a tricky one / hard to know.
I would say it applies only to the interest that has been applied to the account and has had DIRT deducted from it. Good if your interest is applied monthly, bad if it is yearly.