I fly from STL, but I generally don't go straight to Dublin, way too pricey. I find that it is (ironically) much less expensive to fly to London and then double back from there.
Our last trip cost us $230 rt to London on BA (STL-ORD on AA) then an add'l $60 r/t from London to Dublin on BMI. However, that was before the price of fuel started skyrocketing. Sales to anywhere in Europe from the US are really rare now, though Aer Lingus did have a few sales to Ireland this year. I'd watch Aer Lingus. There is also an agency called O'Connor Fairways out of NYC that specializes in Irish travel and offers charters that can be a bargain. My extended family has been using them regularly since the 60's, and they are consistently well-priced. Ireland is one of the few places that actually has a lot of decent charters out of the U.S.
I don't know how comfortable you are with coach seating, but sometimes it is really worthwhile to take a domestic flight to the East Coast and take your over the water leg from there; we find that going from Europe all the way to the Midwest while stuck in an int'l coach seat is a couple of hours more torture than we prefer to endure. It's not so bad E-bound because you will sleep, but W-bound is a bear.