I'll put in a plug for the North -- the Co. Antrim coastline is absolutely gorgeous. Giant's Causeway is a natural wonder that is indeed an interesting & amazing sight. There is a particularly nice ruin, IME, called Dunluce Castle that isn't far away. And the Bushmills Distillery is also conveniently in the area and has a nice tour/tasteing.

Another plus is that it isn't as heavily touristed as the Republic. Things can get tense in July with the Marching Season, but mostly I wouldn't hesitate for a second to go to Northern Ireland.