Carrickfergus ( ˌkaːɾɣəɟ ˈaɾɣɣəʃ, meaning "Fergus' rock") is a large town in County Antrim, Northern Ireland. It sits on the north shore of Belfast Lough, from Belfast. The town had a population of 28,141 at the 2021 Census. It is County Antrim's oldest town and one of the oldest towns in Ireland as a whole. Carrickfergus Castle, built in the late 12th century at the behest of Anglo-Norman knight John de Courcy, was the capital of the Earldom of Ulster. After the earldom's collapse, it remained the only English outpost in Ulster for the next four centuries. Carrickfergus was the administrative centre for Carrickfergus Borough Council, before this was amalgamated into the Mid and East Antrim District Council in 2015, and forms part of the Belfast Metropolitan Area. It is also a townland of 65 acres, a civil parish and a barony.
The town is said to take its name from Fergus Mór (Fergus the Great), the legendary king of Dál Riata. According to one tale, his ship ran aground on a rock by the shore, which became known as "Carraig Fhearghais" – the rock of Fergus.
As an urban settlement, Carrickfergus far pre-dates the capital city Belfast and was for a lengthy period both larger and more prominent than the nearby city. Belfast Lough itself was known as 'Carrickfergus Bay' well into the 17th century. Carrickfergus and the surrounding area was, for a time, treated as a separate county. The historical walled town originally occupied an area of around 97,000 square metres, which now comprises the town centre, bordered by Albert Road to the west, the Marine Highway to the south, Shaftesbury Park to the north and Joymount Presbyterian Church grounds to the east. Segments of the town wall are still visible in various parts of the town and in various states of preservation. Archaeological excavations close to the walls' foundations have yielded many artefacts that have helped historians piece together a picture of the lives of the 12th and 13th century inhabitants.