kayak.com is your friend.
Put in every possible city pair in the world. For example, I flew roundtrip NYC to Dusseldorf for $750 this summer. At one time there were cheap fares to LAX as well. So maybe you could buy an advance purchase train ticket to Dusseldorf, fly to LAX, then take the train to San Diego.
There are also very cheap fares to Ireland from the U.S., so maybe it will be the same on the reverse.
Of course, it might be cheaper to go from London. Just giving you some thoughts.