I work for O2 at their HQ and I'm fully aware of customers upset of large bills... sadly it happens frequently!
The EU recently set the standards for the calls and these are 35ppm for calls 18ppm to recieve calls and 16p for texts.
O2 at the moment are charging the above costs, however if your an O2 customer you get a thing called the Ireland Bolt On which reduces the cost to 25ppm. We also don't charge you anything for your texts, so say for example you are given 500 texts messages, we take 4 texts from the allowance for every one text you send. You aren't charged anything for recieving them (worldwide).
If you want to keep your number, best thing to do is ask T-Mobile for a "PAC" code and you can transfer your number to any network free of charge.