If you want the cheapest flights possible, start in Dublin.
7th April - Dublin to NY direct
13th April Las Vegas to Dublin via London....£333. (3 nights in each or 2/4 split)
Factor in around £100 for your domestic one way, and your flight to Dublin start at £9.99 on Ryanair. Take hand luggage if possible, and then hop off in London on your return and don't bother taking the final leg back to Dublin.
Leaves more money to gamble with