Ireland: 1627 to 1642
In March 1627, St Leger joined the Privy Council of Ireland and succeeded Buckingham's half-brother Sir Edward Villiers as President of Munster. Along with this position, he inherited Buckingham's struggle for control of the province with his local rival Richard Boyle, 1st Earl of Cork, and to counter this, St Leger became an ally of Thomas Wentworth when the latter was named Lord Deputy of Ireland in January 1632. In 1630, St Leger had proposed a new plantation scheme in County Tipperary, which was never implemented but aligned with Strafford's long-term policy of expanding Protestant cultural and religious dominance in Ireland.
However, in the short term, Strafford was less concerned with religious issues and more focused on increasing royal income by reclaiming lands that rightfully belonged to the Crown and ending widespread corruption among government office holders. Both particularly impacted so-called New English settlers like Boyle, who were the primary beneficiaries of such practices and viewed Catholicism as an immediate threat to their positions. When St Leger was elected Member of Parliament for County Cork in the Irish Parliament of 1634–1635 where he backed Strafford's policy of making limited concessions to the Catholic gentry in return for subsidies.
The first of the Bishops' Wars in 1639 ended with the Covenanters in control of Scotland, a defeat Charles I was determined to reverse. To help achieve this, in December 1639 Stafford called the 2nd Irish Parliament of King Charles I to approve funding for an Irish expeditionary force that would land in Western Scotland. St Leger was re-elected for County Cork to the Irish Parliament of 1640–1649. Under Strafford's guidance, Parliament unanimously voted four subsidies of £45,000 to fund an Irish army of 9,000 for use against the Scots in the Second Bishops' War.
St Leger was appointed Sergeant Major with responsibility for training the new recruits, who were based in Carrickfergus ready for transport to Scotland or northern England. In July 1640, St Leger declared them ready for service but they remained in Ireland throughout the Second Bishops' War. The main impact was to deepen divisions between Strafford and his opponents in both the Irish and English Parliaments, which led to his trial and execution in May 1641.
An additional complication was what to do with the newly formed army, many of whom were Catholic and thus viewed with deep suspicion by the Protestant-dominated Dublin Castle administration. It was eventually agreed they could take service with Spain and when the Irish Rebellion began in October 1641, around 2,500 men under Garret Barry were waiting in Kinsale for shipment to the Army of Flanders.
As Lord President, St Leger was responsible for dealing with the insurgents in Munster, but the forces and supplies placed at his disposal were inadequate. In February 1942 he was reinforced by the arrival at Youghal of Charles Vavasour and his regiment. He also brought St Leger the royal declaration of 1 January 1642 in which the King denounced the rebels. St Leger was still struggling with the insurrection when he died on 2 July 1642, either at Doneraile Court or at Cork.