He was the son of Sir Warham St Leger and his wife Elizabeth Rothe of Kilkenny, grandson of William St Leger and a great grandson of Anthony St Leger, Lord Deputy of Ireland. He was probably born in County Cork. When he was 14 his father and Hugh Maguire fatally wounded each other in single combat near Cork city. He took part in the Flight of the Earls in 1607, when Hugh O'Neill, Earl of Tyrone, and Rory O'Donnell, 1st Earl of Tyrconnell, together with more than ninety of their family and followers, the chief of the Gaelic and Catholic resistance in Ireland, fled to Europe. He later said that his involvement in the Flight was accidental, in that he had sought the protection of Tyrone from legal proceedings being taken against him, and fled with him because he had nowhere else to go. Sir William spent several years abroad, living firstly in Brussels and then Dordrecht, where he married his first wife Gertrude de Vries. Having received a pardon from King James I and extensive grants of land in Ireland, he was appointed President of Munster by Charles I in 1627. He warmly supported the arbitrary government of Thomas Wentworth, Earl of Strafford, actively assisting in raising and drilling the Irish levies destined for the service of the king against the Parliament. He was a member of the Irish House of Commons from 1634, as MP for County Cork. In the great Irish Rebellion of 1641 he bore the chief responsibility for dealing with the insurgents in Munster; but the forces and supplies placed at his disposal were utterly inadequate.
Reputation
His reputation in the minds of Irish nationalist historians is that he executed martial law in his province with the greatest severity, hanging large numbers of rebels, often without much proof of guilt. In 1843 Daniel O'Connell quoted him as saying about the harsh policy adopted by the government in Dublin: "The undue promulgation of that severe determination to extirpate the Irish and papacy out of the kingdom, your Lordship rightly apprehends to be too unseasonably published" in a such sense that he approved of the policy of extirpation. O'Connell went on "This St. Leger was himself one of the chief extirpators". The quotation can also be read in another sense, in that St Leger's use of the words "undue", "severe" and "too unseasonably" point to his disapproval of such a policy. As a landowner, St Leger's Irish property would have been worthless without Irish labourers and tenant farmers to work on it. In terms of geography, his reputation for cruelty in Munster is actually limited to a small area near Clonmel in early December 1641 where his force did kill several dozen people without due process, which led on to the killings in revenge of English or Protestant people in the same area. Whether St Leger ordered or approved of the murders by his force is still unproven. Why he would want to provoke such an uprising in Munster, where 90% of the population was Irish and Catholic, and where Protestants like him numbered less than 10%, is still unclear. He was still struggling with the insurrection when he died at Cork in 1642. William married twice and had 6 children. Sir William's daughter Elizabeth, by his first wife Gertude de Vries of Dordrecht, married Murrough O'Brien, 1st Earl of Inchiquin; his son John was father ofArthur St Leger, created Viscount Doneraile in 1703.
