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. He was also made a privy councillor in Ireland, and received a grant of lands within the Pale.
This event created a deep impression in Ireland, where O'Neil's submission to the English king, and his acceptance of an English title, were resented by his clansmen and dependents. The rest of the earl's life was mainly occupied by endeavours to maintain his influence, and by an undying feud with his son Shane, arising out of his transaction with Henry VIII. For not only did the nomination of O'Neill's reputed son Matthew as his heir with the title of baron of Dungannon by the English king conflict with the Irish custom of tanistry, which regulated the chieftainship of the Irish clans, but Matthew, if indeed he was O'Neill's son at all, was illegitimate; while Shane, Conn's eldest legitimate son, was not the man to submit tamely to any invasion of his rights. The fierce family feud only terminated when Matthew was murdered by agents of Shane in 1558; 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 illegitimate daughter of Conn married the celebrated Sorley Boy MacDonnell.
{| border="2" align="center"
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|width="30%" align="center"|Preceded by:
New Creation
|width="40%" align="center"|Earl of Tyrone
|width="30%" align="center"|Followed by:
Hugh O'Neill
|}
Reference