In Ireland and the UK you can fly on an Irish/UK registered aircaft without any validation (involvement of the IAA/CAA) as a PPL VFR only so regardless of the license you hold (CPL,ATPL)
To work commercially you would need a validation of your foreign CPL/ATPL, which can only be applied for by an operator, has certain experience levels attached and would be valid for 1 year and can officially not be extended. These validation are rarely given to helicopter pilots unless they have tons of experience and the operator can't get a JAA licensed pilot. You would also have to do an Airlaw exam and flighttest.
A foreign student pilot license/certificate is NOT valid in Ireland. A FAA instructor cannot give flight instruction in Ireland/UK unless also JAA FI and working through a RTF or FTO.