Doing the same trip later this year, but personally, I haven't found the travel agents competitive. If you're up to doing it yourself, try the following:
1. Flights: Delta.com seems to be the most competitive for that kind of trip at the moment. If you do a "multi-flight" option on their website, from Ireland (Dublin or Shannon), on to JFK, then on to Vegas, and return the same way, your flights should come in at 750-800.
2. Accommodation: start off with hotels.com (check the reviews on tripadvisor.com) and price your hotels in Vegas and New York. Then try the website of your chosen hotel - you might get an equal or better price. Finally, call them directly and validate everything - you might even get a better deal over the phone. For Vegas, stay on the Strip, south end (near MGM). It's the better end for first timers. For Manhattan, there's lots of hotels in the Midtown region (there's another thread on this about where is best to stay in NY). Target $130 or so for your room in Vegas, and $200 in NY to get a reasonable standard.