Sir Turlough Luineach O'Neill (Irish: Toirdhealbhach Luineach mac Néill Chonnalaigh Ó Néill) fostered by the O'Lunaigh family; son of Neill Chonnalaigh O'Neill) (1532 – September, 1595), was an Irish Gaelic lord of Tyrone in medieval Ireland. He was inaugurated upon Shane O’Neill’s death, becoming The O'Neill.
Turlough was born around 1530 at Seanchaisleán close to the modern town of Newtownstewart. He was the fourth son of Niall Connallach macArt óg O'Neill, Tanist of Tyrone (1519–1544). As Tanist, Niall Connallach was designated to succeed his uncle Conn Bacach mac Conn, The O'Neill (1519–1559). Turlough's mother may have been Niall Connallach's wife, Rose O'Donnell, the daughter of Manus, The O'Donnell of the kingdom of Tyrconnell. Turlough was the grandson of Art óg macConn, The O'Neill (1513–1519) and was a direct descendant of Brian macNéill Ruad, The O'Neill and ruler of Tyrone (1238–1260).
Making professions of loyalty to the Queen of England in the year following Shane's assassination, Turlough sought to strengthen his position by alliance with the O’Donnells, MacDonnells and MacQuillans. But his conduct giving rise to suspicions, an expedition under Walter Devereux the earl of Essex was sent against him, which met with such doubtful success that in 1575 a treaty was arranged by which O’Neill received extensive grants of lands and permission to employ three hundred Scottish mercenaries. A further treaty in 1578, negotiated by Lady Agnes, confirmed Turlough’s vast land holdings in Ulster, granted him a Knighthood and the British titles of Earl of Clanconnell and Baron of Clogher, for life, and allowed him to retain for life the personal army of Scottish mercenaries negotiated three years before.