Elizabeth O'Donnell (1604 – c. 1630) was the 1st Countess of Fingal. She was the daughter[1] of Prince Rory O'Donnell, 1st Earl of Tyrconnell, and Lady Bridget FitzGerald, daughter of Henry FitzGerald, 12th Earl of Kildare. After her father's death in 1608, her mother married Nicholas Barnewall, 1st Viscount Kingsland, and had more children. Meanwhile, Elizabeth married Luke Plunkett, 1st Earl of Fingall[2]. They lived at Killeen Castle, County Meath in Ireland.
Given her probable very young age, some sources suggest that this Elizabeth may have been a FitzGerald, the Countess Lady Bridget’s sister , and not her daughter, however no evidence is given [3] She may have been a child-bride, and is said to have died in London very shortly after her marriage. Her lineage as daughter (possibly pre-marital) of Rory O'Donnell, 1st Earl of Tyrconnell, and his wife Lady Bridget FitzGerald, has been repeatedly cited in various peerage directories, such as Lodge's (1827, and re-issued in 1907)[4], Foster's (1881)[5], Cokayne's (1926)[6], and Burke's (1980)[7].
Lord Fingall was also married on three other occasions. He had married in 1628 Eleanor Bagenal, who died leaving no children. His son by a later marriage to Susanna Brabazon, Christopher Plunkett, 2nd Earl of Fingall, married Mabel Barnewall, half-sister of Elizabeth. Thus Elizabeth was 1st Countess of Fingall, and her much younger half-sister became Countess of the 2nd Earl of Fingall. Another half-sister, Mary Barnewall married Nicholas Preston, 6th Viscount Gormanston, whereby Elizabeth was both his half-sister-in-law, and already his 3rd cousin through their common ancestor, Gerald FitzGerald, 9th Earl of Kildare. She was also aunt of Jenico Preston, 7th Viscount Gormanston, who was a 1st cousin of Luke Plunkett, 3rd Earl of Fingall, also her nephew. Lucas Plunkett's last wife was Margaret St. Lawrence, daughter of the 8th Baron Howth. Plunkett died 29 March 1637; Margaret died the following November.