Her Wikipedia article follows:
"Katherine Manners, Duchess of Buckingham, 19th Baroness de Ros of Helmsley (died October 1649), also known as Catherine, was the daughter and heir of the 18th Baron de Ros. She was known as the richest woman in Britain, apart from royalty. She married George Villiers, 1st Duke of Buckingham, the favourite, and possibly lover, of King James I of England; and secondly, she married the Irish peer Randal MacDonnell, 1st Marquess of Antrim and was briefly engaged with the Irish Catholic rebels.
Lady Katherine Manners was the daughter of Francis Manners, 6th Earl of Rutland. She married George Villiers, 1st Duke of Buckingham and was the mother of Mary Villiers, Duchess of Richmond and of George Villiers, 2nd Duke of Buckingham (who inherited her title as 20th Baron de Ros). Katherine's first husband, Buckingham, was murdered in 1628 by John Felton. On the death of her father in 1632, without heirs male, she succeeded suo jure to the ancient barony of de Ros. In 1635 she married Randal McDonnell, Earl of Antrim, and went to live at Dunluce Castle, County Antrim, Ireland.
Following the Catholic uprising in Ulster in 1641 the McDonnell family moved south to Wexford, then Waterford, where Katherine died in 1649."