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 in the Glens of Antrim to his north. In 1568, Brian McPhelim 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. Whilst the Clandeboye O'Neill's 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 set about razing any buildings he could find throughout the northern Ards peninsula that could provide shelter. In 1573, a similar scheme for the plantation of County Antrim by Walter Devereux, 1st Earl of Essex, overtook Smith's grant, and eventually saw Smith cede his claims to north Clandeboye to Essex. Despite this, Brian McPhelim 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 Brian McPhelim. 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
It is alleged that Essex, who had been invited to Brian McPhelim's castle in Castlereagh, near Belfast, for dinner, had Brian McPhelim, his wife and brother captured, and murdered between 100 and 200 attainders in attendance in front of their eyes. After that Brian McPhelim, and his two relations were taken to Carrickfergus Castle and executed. In 1574, Brian McPhelim was hanged for opposing the local plantations.
Division of Clandeboye
After his death, Essex promoted Brian McPhelim's son-in-law, Neill McBrian Fertagh O'Neill, son of Brian Fertagh O'Neill who was a cousin of Brian McPhelim, to the lordship of Clandeboye regardless of the other claimants. The inter-familial disputes that arose from this resulted in Lord-Deputy, Sir John Perrot, dividing Clandeboye between the competing members of the Clandeboye O'Neill clan in 1584: Shane McBrian O'Neill, Brian McPhelim's son, received three-quarters of north Clandeboye; Hugh Oge O'Neill, son of Brian McPhelim's brother Hugh, received a quarter of north Clandeboye, centred on Edenduffcarrick; Con McNeill O'Neill, Neill McBrian Fertagh's uncle and Brian McPhelim's cousin, was granted all of south Clandeboye, afterwards known as Upper Clandeboy. North Clandeboye would become known as Lower Clandeboye. The arrangement in Lower Clandeboye led to bitter strife between the first cousins, Shane and Hugh Oge, resulting in the killing of the latter in 1586. After this Neill received Hugh Oge's portion of Lower Clandeboye.
Fate of Clandeboye
Despite Brian McPhelim's attempts to thwart English settlement of his lands, the son of Neill McBrian Fertagh, Con, who succeeded his father as lord of Upper Clandeboye, made a deal with Hugh Montgomery and James Hamilton that resulted in the three-fold division of the lands comprising his estate in 1605. He sold off the rest and died in 1619. Their subsequent plantations expanded rapidly. The scheme for settling Antrim including the territory of Lower Clandeboye passed from Essex to Sir Arthur Chichester, who was more successful than his predecessor.
