You can't bring US pensions back to Ireland, it has to stay in America. Likewise, you can't bring an Irish pension over there. Whether it is worth your while taking one out depends on how long you intend to stay. The longer the stay, the more money you are leaving on the table.
You can also set up a US investment account over there that are cheap as chips. something like a betterment or vanguard account.
Steven
http://www.bluewaterfp.ie (www.bluewaterfp.ie)