I haven't personally flown the DUB-PHL route, but Dublin offers US CBP preclearance. You may do well to call US Airways in Ireland and verify this.
http://www.gadling.com/2008/11/18/ir...d-immigration/
This site says that immigration is done in Dublin and customs in America. In any case, the portion you are concerned with (the green waiver form) will certainly be handled by the immigration preclearance folks in Dublin. So, if something goes wrong, at least you are still in Ireland and not stuck in Philadelphia.
In any case, clearing customs without queueing up in an immigration line should still take well under two hours, especially coming in on the smaller jet (the US 757).