The site of Grannagh Castle has had strategic significance since pre Christian times.  The first fortress was a Celtic Iron Age Fort or Dun, built around the time of Christ.  It was known as Dun Bhrain.  Dun Bhrain was obliterated when the present castle was built 650 years ago.  The site is so significant because it was the location for the ferry crossing linking Waterford to the other major medieval cities of Kilkenny Limerick and Dublin

The riverside location of the castle also controlled the riverborne traffic from Waterford to the bustling towns of Carrick-on-Suir, Clonmel and Cahir.  The battlements of the castle were only a few yards from the river channel that the boats used.

Grannagh Castle is a typical Norman style castle. It consists of an enclosed courtyard surrounded by curtain walls, which were adorned by battlements.  In each corner stood a projecting circular, or drum tower, which had loopholes giving clear views of the connecting walls.  In the North East corner stands the main "keep" or "donjon", the most heavily fortified section of the castle.

Following the Norman invasionin Ireland in 1169, all of County Waterford and the southern part of Kilkenny was granted to the de la Poer, or Power family.  This family built the castle in the mid 1300's.  Later in the same century the Powers went to war with the City of Waterford.  This agression incurred the King of Englands displeasure.  His allies the Butlers of Ormonde joined the war against the Powers.  The Powers lost the war and were forced to surrender South Kilkenny and Grannagh Castle to the victorious Butlers.

James Butler, the third Earl of Ormonde, took possession of the castle circa 1375.  It became an important possession of the Butlers and the administrative centre for the extensive manor of Grannagh, which extended over thousands of acres along the north bank of the river Suir.  For the rest of the Middle Ages, Grannagh Castle was the first line of defense for the Butler lands against their hereditary enemies across the rive, the Powers.

Pierce Rue Butler, the Eigth Earl married Mairead Ni Ghearoid, the daughter of the Eight Earl of Kildare.  She was known as the Countess of Grannagh, and had a reputation as a very stern lady with a vicious streak.

In bad temper one day Mairead summoned her jester and ordered him to make cheer her up.  After several jokes and tricks which failed to entertain her, the jester suggested a rope trick.  He quickly fashioned a rope with several nooses.  She responded by demanding to see the invention work.  She ordered her soldiers to round up several peasants and warned the jester that if the invention was not successful, he would hang!  Seven innocent peasants were hung from the battlements, an actrocity which restored the Countess'good spirits.  So delighted was she with "the Butler Knot" that she incorporated it into the family coat of arms.

The castle was last occupied in 1650 when Oliver Cromwell, the English General sacked it and removed the Butler dynasty.  Today the remains of the castle stand on the majestic river Suir opposite the Thatch Pub in Grannagh.