[1] Mabel was born on an unknown date sometime after 1598 in Coleshill, Warwickshire, England, the eldest daughter and one of the ten children of Sir Robert Digby and Lettice FitzGerald, suo jure 1st Baroness Offaly.
He was the son and heir of Sir John Og FitzGerald, Lord of Dromana and Decies, and Elinor Butler.
[3] They made their principal residence at Dromana Castle, County Waterford, and together had three children: Upon the death of her husband on 6 August 1643, Mabel married secondly Donagh O'Brien of Arragh.
Although her husband sided with the English, Mabel showed herself sympathetic to the Irish rebels and in 1642 entertained them at Dromana Castle serving them "beefes, muttons, bread and beere".
The FitzGeralds had managed to keep their estates intact during the Cromwellian settlements due to their Protestant religion and the influence of Mabel's Parliamentarian son-in-law Richard Franklyn, who served as a major in Oliver Cromwell's army.