In very rough terms...
It will be 11% per year of the amount you borrow.
It is recalculated every day. The daily interest rate is 11%/365 or about 3 cents per €100 (if I have my decimal points in the right place.)
So if you run up an overdraft tonight of €1,000 , you will be charged 30 cents interest tonight.
If you clear it tomorrow, you won't be charged any interest tomorrow.
So the right strategy is to keep your actual negative balance in the bank as low as possible.
If you have a facility of €10,000 and don't actually use it you won't be charged interest. You may be charged an annual facility fee. For example, I think AIB charges me €25 a year.
Brendan