We are definitely going to Newgrange/Knowth, but I was wondering if going to either Lough Gur or the Carrowkeel Passage Tomb would be worth it?
We went to both last April. Carrowkeel up by Sligo was well worth the time. Lough Gur wasn't nearly as evocative. There were also several excellent sites in County Donegal - if that interests you, I will look up their names.
If you're half-serious and want to get away from guidebooks and go straight to the encyclopedia: Megalithic Portal.