Best reasons to fly from Ireland;
1/ Most often cheaper to fly to Dub with a low cost carrier and then to the US with Aer Lingus, that is if you don't live in London.
2/ Complete immigration in Ireland, no long queues in JFK or wherever else.
3/ And best of all you don't have to deal with the UK security problems, Dublin is a pain as an Airport but anything is better than Heathrow and Gatwick in my experience.
Lunar