Trim Castle was Ireland's largest castle and it was the centre of Norman administration for the 'Liberty of Meath'. This was one of the new administrative areas of Ireland created by Henry II of England and granted to Hugh De Lacy in 1172. De Lacy built a huge ringwork castle defended by a stout double palisade and external ditch on top of hill beside the river Boyne. The ringwork was attacked and burnt by the Irish but De Lacy immediately rebuilt it in 1173. His son Walter continued rebuilding and the castle was completed c 1204.
Part of the movie Braveheart was made at Trim Castle.
Reference: Wikipedia