County Kildare: See
In Kildare see the Irish National Stud Farm, which contains the Japanese Gardens.
Other gardens are at Larchill near Kilcock, west of Maynooth.
Castletown House is a grand Palladian mansion in Celbridge near Maynooth. It has two remarkable follies, though nowadays they're severed from the house by M4.
Castles: lots of these, but tumbledown or smashed up. Find them at Athy, Maynooth, Carbury and Naas.