O'Neill (Ó Néill), Sir Brian (c.1520–74), lord of Lower Clandeboye (Clann Áodha Buidhe Íochtar), was probably eldest son of Feidlimidh Bacach O'Neill, lord of Clandeboye. He became lord in 1556, at a time when Clandeboye faced terminal decline as a result of internecine dynastic feuding, the looming threat of Shane O'Neill (qv), lord of Tyrone, and an influx of Scottish MacDonnell settlers into the Glens of Antrim. In order to protect his lands, Sir Brian looked to the government for support and sided with them against Shane and his sometime MacDonnell allies. For his service he earned glowing praise in state correspondence to the queen in 1567. Occasional tension arose between Brian and the government; having preserved most of his lands against the odds, he naturally guarded his traditional rights zealously against officials who wished to levy cess in the form of a cattle tax on the lordship. Generally, however, relations were good, as displayed by his warning to the garrison of Carrickfergus of the treacherous intent of Turlough Luineach O'Neill (qv), lord of Tyrone.
However, there was another side to Sir Brian's loyalty. Under his loyal guise, he and his father-in-law, Brian Carrach O'Neill, pursued their own private war with Turlough Luineach, attacking his son-in-law Ruaidhrí Óg MacQuillan (Mac Uidhilín) in the early months of 1568. This act led to a rebuke from the government and fanned suspicions of his intentions. Both Sir Brian and his father-in-law tried to excuse themselves, but the doubts remained and increased when he met Sorley Boy MacDonnell (qv) in February 1571. Sir Brian's growing discontent was revealed in a letter of March, when he complained to the lord deputy of an influx of English settlers into his lordship, and he applied to the queen for royal confirmation of his lands in July 1571.
Sir Brian's fears were confirmed when Sir Thomas Smith (qv) was granted O'Neill lands in the Ards peninsula late in the year. On 3 January 1572 the local English commander warned that this grant would drive Brian into rebellion. Surprisingly, on learning of Smith's grant Brian remained calm and wrote to the queen reminding her of his service. It was said that her reply greatly reduced the building tension, but Smith's arrival in Carrickfergus raised the stakes (August 1572). Brian, with his new ally Turlough Luineach, went on the rampage through the Ards in the winter of 1572–3, destroying the settlements of the colonists. The war continued till Brian's submission in September 1573.
The political landscape changed dramatically when Walter Devereux (qv), 1st earl of Essex, obtained a royal grant of Clandeboye along with a private loan from the queen. He set out to colonise the region thoroughly, but his campaigns against Sir Brian and his allies ended in ignominious defeat. The only glimmer of hope was the latter's submission on 26 April 1574, and his new willingness to cooperate in return for more favourable terms. However, Sir Brian's close alliance with Turlough Luineach made Essex uneasy. With this in mind, he treacherously invited Sir Brian and his followers to a banquet in Belfast in October 1574. In the middle of the festivities, his soldiers burst through the doors and slaughtered all of Sir Brian's followers. Essex dispatched Sir Brian, his wife and his brother to Dublin for trial, where they were publicly executed (November).
Sir Brian married first the daughter of Sir Arthur Magennis, Viscount Iveagh, and secondly Anne, daughter of Brian Carrach O'Neill. He was an able and intelligent man, but unfortunately for him the ruthlessness of Essex and of Brian's former English patrons knew no bounds.