No, you need the permit to stay out of the country for a year.
The requirements of the greencard is you have to be in the country over 6 months of the year.
The re-entry permit will allow extra time away. It can take 6 to 8 weeks to get the re-entry permit when you apply for it so you would have to go to the states anyway for that time to get it after you have first gone on your activation trip and waited for and received the greencard.
Anyway, I wouldnt even bother thinking about it until you are actually picked.
PO Box wont do for a bank account. If you have a relative there that will do. You will need to sort out your social security number etc first as well. Opening a bank account (and one without massive fees) is a lot more difficult than you think after moving over.
Also, bear in mind that if you do activate it and go back to ireland for a while you will still need to do tax returns in the US (and Ireland) for that tax year. And you will have to pick the state you sent your greencard to since you wont be resident in any state yet - but you are considered a resident for the year for tax purposes as soon as you get the greencard. You will probably have to pay some taxes on your irish income in that time.