I actually passed this place after taking a wrong turn. The remaining keep is quite nice and there are picnic benches for your enjoyment.
The place is called Threecastles but a sign tells you that they don't know where the other two were.
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_From Blessington follow the road north around the reservior. The castle is on the right after about 3km. You can't miss it.
A Random Selection of Nearby Monuments
 Ferns (Co. Wexford) |  Caucestown (Co. Meath) |  Castlelost West (Co. Westmeath) |
 Sutton Castle (Co. Dublin) |  Moiry (Co. Armagh) |  Cusackstown (Co. Meath) |
