The castle is unique Norman fortress built on the site of an older  fort by John de Courcy in the 1180s, it took several years to complete due to continual fighting as he pushed his frontiers of power to the north-west as far as the River Bann at Coleraine. De Courcy was a Norman knight who had marched from Dublin in 1177 with the purpose of establishing his own territory in Ulster and expanding Norman interests in Ireland. With 25/30 armoured horsemen and 300-foot soldiers, he captured the main settlement of Downpatrick, secured the strategic crossing at Belfast and took Carrickfergus which was the main strategic town and port in Ulster after Downpatrick.

He made Carrickfergus his stronghold and from here expanded his power base, De Courcy founded St Nicholas Church in 1192 and Woodburn Priory. The well-preserved castle guards the entrance to Belfast Lough and would have originally been surrounded by water. Over its history it has served as a stronghold for numerous governors and conquerors, and has been besieged by French, Scots, English and Irish troops. Carrickfergus remained in the ownership of De Courcy until 1203 when he was overthrown by Hugh De Lacy with the blessing of King John of England.

Hugh De Lacy became the First Earl of Ulster in 1205, this was the first creation of earls, there would be a second creation and another 1st Earl of Ulster in 1271. De Lacy lost the castle in 1210 when King John arrived and forced him to flee to Scotland and then to France after a war broke out between the Earl and FitzHenry, the King's Judiciary in Ireland. The castle returned to Hugh De Lacy in 1227 after Henry III became king. De Lacy then added the gatehouse, towers and outer walls and also endowed the Franciscan Friary, he lived here until his death in 1242.