I have done 2 Transatlantic Cruises, but none using the north atlantic crossing which NY to Ireland would be. The crossing in April from Miami to Barcelona uses the southern Atlantic route and on both occassions apart from the very odd occassion the seas were quite calm and comfortable.
The 6 days at sea depends on you!. Personally I like it because unlike most cruises you dont have to get up early and rush around to take tours at various Ports of call. Its very leisurely - get up when you want, eat when you want and do as little or as much as you want on the ship. If you are very active you might find the 6 days a bit much. Either way you get 6 days to get gradually adapt to the 5 hour time change...that makes a big difference when you are going to Europe. Enjoy!