The town's name is a small piece of mythology. *Carraig Fhearghais* means "the rock of Fergus", and the Fergus in question is Fergus Mór, the legendary king of Dál Riata, the Gaelic-speaking sea kingdom that bridged northeast Ireland and western Scotland in the early Middle Ages. According to one tale his ship struck the rock by the shore and he died on it. Whether it happened or not, the name stuck. When the Anglo-Norman knight John de Courcy invaded Ulster in 1170 and looked for somewhere to plant a castle, he chose Fergus's rock. By 1177 he had begun building Carrickfergus Castle on it. The castle is widely considered one of the best-preserved Norman castles in Ireland. It has been continuously occupied or in military use for nearly all the centuries since.

After the Earldom of Ulster collapsed in the fourteenth century, Carrickfergus had a peculiar status: it was the only English outpost in Ulster, a single fortified town in a province otherwise held by Gaelic chieftains. It held that position for four centuries. The town walls went up under Sir Arthur Chichester at the end of the 1500s, by which time English and Scottish settlers were being planted in the surrounding country. The Presbytery of Carrickfergus, the first in Ireland, was set up here in 1642. In 1689 the castle held out for several days against William of Orange's besieging forces before surrendering on 28 August. William himself landed at Carrickfergus harbour on 14 June 1690 - en route to the Battle of the Boyne - and the plaque marking the spot is still on the quay.

On 24 April 1778, the American Revolutionary War briefly came to Belfast Lough. John Paul Jones, the Scottish-born father of the United States Navy, sailed the Ranger up the lough hoping to capture HMS Drake, a British Royal Navy sloop moored at Carrickfergus. The first attempt failed. He came back a few days later, drew the Drake out into the North Channel, and beat her in open water - one of the few American naval victories of the war. Eighteen years earlier, in February 1760, the town had been briefly captured altogether by a French naval squadron under François Thurot during the Seven Years' War. The defenders ran out of ammunition and were held to ransom. Carrickfergus was, for a small Ulster town, a remarkably attractive target.

In 1711, Carrickfergus Courthouse heard the last witchcraft trial in Ireland. Eight women were accused of bewitching a young girl named Mary Dunbar; all were convicted, despite one of the judges effectively telling the jury to acquit. The sentence was a year in prison and four sessions in the pillory. A wooden mock-up of that pillory now stands in the town centre as a tourist attraction, which is a peculiar way to remember a miscarriage of justice. In 1797 the same courthouse hosted what was widely considered a show trial of the United Irishman William Orr, who was hanged for sedition on 14 October. The following year, on the outskirts of town, fellow United Irishman Henry Joy McCracken was captured while trying to escape to America. He too was hanged. The phrase "Remember Orr" became a rallying cry for the 1798 rebellion.

On the morning of 2 April 1912, thousands of Carrickfergus residents lined the shore to watch a black hull move slowly up Belfast Lough on its way out to sea. It was the Titanic, on her sea trials, anchoring overnight off the town before continuing south. Twelve days later she would be at the bottom of the Atlantic. In 1909, three years before that anchorage, a two-year-old boy named Louis MacNeice arrived in Carrickfergus when his father was appointed Rector of St Nicholas's Church of Ireland. He left for boarding school in England at ten. His 1937 poem *Carrickfergus* is one of the great twentieth-century Northern Irish poems, looking back on the town with ambiguous affection - "I was the rector's son, born to the Anglican order" - and the Norman castle, the foundries, and the slow Lough are all in it.