I'm not aware of any cruise line offering much in the way of cruising to Ireland, let alone cruise & stay.
Both Princess & P&O make stops in Ireland (Dublin, Belfast, Cobh for Cork, and, Waterford) en route to other countries. It maybe worth seeing if you can get a flight into ireland for a few days, then joining the ship when she arrives, the flying back to Ireland (or the States) after the cruise.
I really don't think that there's any other options (at the moment).
Last: Oceania Nautica Aug 2016
Next: Cruise Maritime Magellan Nov 2016
Then: P&O Oceana December 2016
Then: NCL Jade August 2017
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(3), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2),
Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey