Early career in Ireland
Belonging to a family which took its name from Courcy, Calvados, John de Courcy, of Stoke Courcy, in Somerset, came to Ireland around the year 1171 as part of the Norman invading forces, brought in as mercenaries working for Diarmaid Mac Murchadha, the ousted King of Leinster, to help him regain his position as king. De Courcy's great-grandfather, Richard de Curci is named in the Domesday Book. His grandfather, William de Courcy I, married Emma of Falaise. His father, William de Courcy II, married Avice de Meschines and died about 1155, leaving the family estates in Somerset and elsewhere in England to his son, William de Courcy III, John's elder brother.
John was very ambitious and wanted lands for himself. He decided to invade the north of Ireland which was controlled by Irish dynasties. In early January 1177 he assembled a small army of 22 knights and 300 foot-soldiers and marched north, at the rate of thirty miles a day. They skirted the back of the Mourne Mountains and took the town of Dún Dá Leathghlas (now Downpatrick) by surprise. After two fierce battles, in February and June 1177, de Courcy defeated the last King of Ulaid, Ruaidhrí Mac Duinnshléibhe.
He did all this without King Henry II's permission.
After conquering eastern Ulster he established his caput at Carrickfergus, where he built an impressive stone castle. Other monasteries and castles that he built are Inch Abbey and Dundrum. He married Affreca, daughter of Godred II Olafsson, King of Mann. It is likely that the marriage, as in the case of many kings and those aspiring to be kings in those days, was political, to seal an alliance with her father who paid homage to the King of Norway. John and Affreca are not recorded to have had any children. Affreca built a monastery at Greyabbey dedicated to Saint Mary of The Yoke of God. She is buried there and her effigy, in stone, can still be seen.
In 1183, de Courcy provided for the establishment of a priory at the cathedral of Down with generous endowments to the Benedictines from Chester in England (free from all subjugation to Chester Cathedral). This building was destroyed by an earthquake in 1245. He also created a cell for Benedictines at St. Andrews in the Ards (Black Abbey) for the houses of Stoke Courcy in Somerset and Lonlay in France, which was near Inishargy, Kircubbin, in present-day County Down. The early Irish monastery of Nendrum was given to the Benedictine house of St Bees in Cumberland in order that they might also establish a cell. His wife, Affreca, founded the Cistercian monastery of Grey Abbey, Co. Down, as a daughter house of Holm Cultram (Cumberland) in 1193.
He also made incursions into the west to increase his territory and lordship. In 1188 he invaded Connacht, but was repulsed and the next year he plundered Armagh.