Conn, who was related through his mother with the earl of Kildare (Fitzgerald), became chief of the Tyrone branch of the O'Neills (Cinel Eoghain) about 1520.
When Kildare became viceroy in 1524, O'Neill consented to act as his swordbearer in ceremonies of state; but his allegiance was not to be reckoned upon, and while ready enough to give verbal assurances of loyalty, he could not be persuaded to give hostages as security for his conduct; but Tyrone having been invaded in 1541 by Sir Anthony St Leger, the lord deputy, Conn delivered up his son as a hostage, attended a parliament held at Trim, and, crossing to England, made his submission at Greenwich to Henry VIII., who created him earl of Tyrone for life, and made him a present of money and a valuable gold chain.
The fierce family feud only terminated when Matthew was murdered by agents of Shane in 1J58; Conn dying about a year later.
Conn was twice married, Shane being the son of his first wife, a daughter of Hugh Boy O'Neill of Clanaboy.
An ille gitimate daughter of Conn married the celebrated Sorley Boy MacDonnell.