Trim Castle was built in the 12 by Hugh de Lacy, a Norman knight sits on the River Boyne.
Trim, a historic town, is about an hour drive north of Dublin and is easy to find. You can look around the grounds outbuildings and take a guided castle keep. I recommend a tour of the castle, and access to the keep is guided tour.
The tour guide told about how life might have been in the castle when it was in use. The most disgusting (or interesting) were the holes in the chambers for a “toilet”, which funneled down into a vat down in the basement, where stirred the vat to release ammonia, which wafted back up the duct to the “toilet”, over which people hung their clothes to let the ammonia get rid of lice.
Some other buildings you can see on the grounds are the Great Hall, where feasts were held and politics discussed River Gate, which allowed deliver supplies from the river; the Curtain Wall; the Barbican Gate, a tower and bridge guarding the southern approach to the castle; and the Solar, previously a tower used for defense now converted to private apartments
The town of Trim has more ruins and a river walk. For example, across a bridge from Trim Castle, there is the Yellow Steeple, a remnant from the Priory of St. Mary, and the Sheep Gate, which is the only remnant from when the entire town of Trim was enclosed within a wall.