Lady Agnes Campbell (1526–1601) was the daughter of Colin Campbell, 3rd Earl of Argyll, and his wife Lady Jean Gordon, daughter of Alexander Gordon, 3rd Earl of Huntly. She was likely born at Castle Campbell which was the Campbell Family seat. Her sister, Elizabeth, married James Stewart, 1st Earl of Moray, an illegitimate son of King James IV of Scotland.
Campbell married James MacDonald of Dunyvaig and the Glens in 1545, forming an alliance with the MacDonalds. They had a daughter named Fionnghuala, or more famously known as Ineen Dubh. Fionnghuala married Aodh mac Maghnusa Ó Domhnaill. Her husband James died in 1565 while he was being held prisoner by the Irish chieftain Shane O'Neill. At that time O'Neill had been supporting the English.
In 1569 Campbell travelled to Ireland to marry Turlough Luineach O'Neill who had replaced Shane O'Neill as the Chieftain of the O'Neill's in Ulster. She took with her a dowry of 1,200 Highland troops, and Gaelic tradition allowed her to lead the troops. She led them against occupying English forces and proved herself a formidable leader. She helped mobilise Scottish support for the Irish. She was fluent in English and Latin which greatly impressed the English.