Some ATMs in Ireland will accept 6-digit PINs. However the majority of them require a 4-digit PIN. The PIN should be numerical as letters may not be displayed on the key pads of the ATMs. You can take a look at these pages for more info: http://www.irelandyes.com/money.html & http://www.irelandyes.com/tips.html
Just got an email from Bank of Ireland reiterating that ATMs in Ireland require a 4-digit PIN. I hope your bank is right. Maybe you should have back-up plans just in case.