The Visa Waiver Program for certain nationals (Irish included) is acceptable, as stated, for persons wishing to enter the US for hour building. Under certain circumstances a B-class visa is acceptable.
The VWP is also acceptable for persons wishing to undertake FAA night rating training or JAR-FCL night qualification training.
Remember to give yourself about 3 to 4 weeks for the FAA to supply a US-airman certificated based upon your IAA-issued JAR flight crew licence.
FAA Part 91 rules available
here. FAA's School Locator database [
link] gives you an abundance of options. AirNav maintains an up-to-date fuel price database [
link].
Best of luck.