The next invaders of Ireland were the Normans, who came to Leinster by invitation, but seized much of Ulster in the next few years.  John de Courcy moved north in 1177 with a small band of 22 horsemen and 300 footsoldiers.  The Normans captured the coastal areas, and built earthen mottes-and-baileys and later stone castles to preserve their conquests.  The greatest of these castles was at Carrickfergus, which was the most important place in the new Earldom of Ulster.  In 1210, the castle was besieged and occupied by King John, being held against him by the rebellious Hugh de Lacy.

Several other towns were founded by the Normans, Coleraine being the most significant.  It was here, in about 1248, that the first bridge across the Bann was built.  The Earls dominated Ulster, and often waged war on the Irish rulers to the west, but their power was broken by the invasion from Scotland of Edward Bruce.   After crushing the forces of the Red Earl, Richard de Burgo in a battle at Connor in 1315, the Scots captured Carrickfergus, after a year-long siege that led the garrison to resort to cannibalism.  Although the Red Earl eventually regained his lands, the murder in 1333 of the Brown Earl, his successor, by some of his own barons, leaving only his two-year-old daughter to succeed him, led to the final collapse of the earldom.

The O’Neills of Clandeboye moved in from Tyrone, and also the MacDonnells, retreating from Scotland where their power as Lords of the Isles had been destroyed, established themselves along the Antrim Coast.  They built castles at Dunluce, at Dunariney, Ballycastle and at Red Bay, the ruins of which may still be seen.  They formed marriage alliances with the leading families of the province, and brought followers with them.  Many galloglasses, mercenary soldiers usually armed with battleaxes, came from Scotland in these times, fighting for Gaelic chieftains but often settling in Ireland.

The O’ Cahans, who were closely allied to the O’Neills, came to dominate much of the modern County of Londonderry, then called O’Cahan’s Country.  They had several castles in the area, their main stronghold being at Limavady, which means “The dog’s leap”, from a legend that a dog belonging to the O’Cahans had leapt over the river gorge here, to warn of a surprise attack.  The Dungiven Costume, found by a local farmer in 1956 while digging peat, may well have been worn by an O’Cahan soldier. It dates from the seventeenth century.