From the recent results it seems that the TA traffic is 50% non connecting, 30% Europe to EI US destinations and vice versa and 20% US connections. Adding a new destination kind of needs to be able to match that profile. EI will have data on which US cities people are booking to/from Dublin so will have a good idea on which city would work best, I think Texas and Flirida are the most likely. One risk is that a new destination may move traffic from other US routes/ connections but Toronto and San Fran doesn't seem to have had an impact.
If I had to guess I would say two more destinations will be announced in the next 8 weeks.