For a start, read our guide on education and schools in Ireland. There are, of course, differences in the school systems of Ireland and America, and a period of transition will be normal for any child.
Your youngest will probably adapt quickly (kids are remarkably resilient), but if you're concerned about your teenager, have you considered international schools? The IB program is internationally acclaimed, and international schools generally provide a smooth transition for expat children. Some also offer the American curriculum.
Hope this helps,