The sites you cant go home without seeing are probably Newgrange/Knowth/Dowth, both hilltops at Loughcrew, Fourknocks, Tara. Next door in Louth you have Proleek dolmen and wedge tomb, and in the border area there's Clontygora, Annaghmare, Ballymacdermot, Ballykeel. All of these sites are within 1 1/2 hours from Dublin. Theres plenty in Dublin to see also, Glendruid, Kiltiernan and Ballybrack, I'm sure Fourwinds will have other suggestions. If I had two or three days here that would be my plan.