I did a quick search on US Airways (PSA), Dublin > Long Beach (stops in Philly), same dates and number of passengers (1 adult 2 kids), it's a 17 hr + flight (vs 11 hrs non-stop Dub > LAX), and came out to 3,224 USD or 1,610stg - yikes! Not only that, check out all these extra charges:
Tax: US International Arrival $15.10
Tax: US International Departure $15.10
Fee: US Immigration $7.00
Fee: US Animal & Plant Health Inspection Svc. $5.00
Fee: US Customs User $5.50
Fee: US Passenger Facility Charge $18.00
September 11 Security Fee $10.00
Fee: Irish Passenger Charge $14.05
Fee: Irish Airport Security Charge $4.74
What is a September 11 Security Fee, and an Irish Passenger charge, do Irish people cost extra?
Haven't looked into Jet Blue yet. There is also supposed to be a flight from Belfast to Las Vegas starting this summer, which might be worth looking into.