I'd like to book a flight transiting Dublin in order to use hidden-city ticketing. How can I find all flights transiting through Dublin from NYC?

One way flights from the US are typically quite expensive whatever the destination, because using two oneway tickets together you can defeat restrictions imposed by the cheaper round trip tickets such as minimum stays or Saturday nights. Therefore twice a oneway fare will usually be at least as expensive as the cheapest restricted roundtrip ticket, although this is not always true.

In any event, you can use the ITA Matrix to construct this kind of search.

I give an example of how to do this in the included screenshot.

Click the Advanced routing codes link. In the new box that appears between origin and destination, enter the routing syntax F+ DUB F+. This means at least one flight, then a transit through Dublin, then at least one flight. For the destination, give a list of European cities. Off the top of my head I gave

LON, BHX, AMS, PAR, MAN, EDI, MAD, BCN, ROM, FRA, STO, OSL

If you want to narrow that list down, look for places served by Aer Lingus on the Dublin airport Wikipedia page, as this carrier is competing with nonstops offered by other carriers. Don't make the list of destinations too long as the Matrix will not check every possible option. Instead, once your list goes above about ten, split it into several searches.

I assume you are happy to make connections before you get to Dublin, but you can change the routing command if you want direct flights. If you only want direct flights to Dublin, use N DUB F+. The N means exactly one non stop flight.

In order to book the itinerary, go to the website of the carrier in question, or try Expedia, or your favourite travel agent, and search for exactly the flights that ITA suggests.