O'Neill was the son of
Phelim Bacagh O'Neill. In 1556 he became lord of Lower Clandeboye. O'Neill sided with the English government in Ireland to help bolster his position against the threat of
Shane O'Neill of Tyrone to the west and a large influx of Scots Highlanders from
Clan MacDonald of Dunnyveg led by
Somhairle Buidhe MacDonnell into the
Glens of Antrim to his north. In 1568, Brian McPhelim O'Neill would be knighted for his service to the Crown as part of
William Piers' campaign against
Shane O'Neill of Tyrone. O'Neill and his father-in-law,
Brian Carragh O'Neill, would wage a private war against Shane O'Neill's successor,
Turlough Luineach O'Neill. The government were not impressed and grew suspicious of O'Neill. In 1571,
Sir Thomas Smith, Queen Elizabeth's principal Secretary of State, was able to get a grant for the entire territory of Clandeboye. While the Clan O'Neill of Lower Clandeboye had been established in that area for two centuries, it had once been part of the
Earldom of Ulster, which upon the death of its last earl in the 15th-century passed into ownership of the Crown. Smith had his son Thomas put in charge of starting a colony and planned to firstly settle the
Ards peninsula and then eventually moving westwards through Clandeboye via a mixture of conquest and plantation. The planned plantation was mishandled especially due to Smith advertising the venture, and Smith went to Carrickfergus to negotiate with Brian O'Neill who was unhappy about the plans. The negotiations failed to happen and Brian McPhelim O'Neill set about razing any buildings (excluding abbeys and priories) he could find throughout the northern Ards peninsula that could provide shelter. In 1573, a similar scheme for the plantation of County Antrim (the south of which was north Clandeboye) by
The 1st Earl of Essex overtook Smith's grant, and eventually saw Smith cede his claims to north Clandeboye to Lord Essex. Despite this, Sir Brian continued to create unrest and disturbances throughout his territory that heavily affected the schemes. Eventually the scheme had to be altered focusing on coastal settlements, however this too failed due to Sir Brian. Eventually Essex had to console himself with a grant for the
Islandmagee peninsula on the east coast of Antrim, to which he was able to successfully plant all the way south to Belfast. These schemes were all part of the
Enterprise of Ulster. ==Death==