When my parents visited, we took them to Trim Castle in County Meath.
Trim Castle is the largest Anglo-Norman castle in Ireland, it was built by Hugh de Lacy when he was granted the Liberty of Meath by King Henry II in 1172. Construction of the massive three storied Keep, the central stronghold of the castle, was begun in 1176 on the site of an earlier wooden fortress. This massive twenty-sided tower, which is cruciform in shape, was protected by a ditch, curtain wall and moat.
Trim Castle was the location for King John’s Castle in the film ‘Braveheart’ the 1995 historical drama directed by and starring Mel Gibson.
Here is a video from Youtube.