O’Farrell appears to have begun his military career serving in one of the regiments of the
Scots Brigade and took part in the 1672 to 1678
Franco-Dutch War. By 1688, he had reached the rank of
Lieutenant Colonel and was part of the invasion force that accompanied William of Orange to England in the November 1688
Glorious Revolution. There was very little fighting; the vast majority of
James II's army simply changed sides and he went into exile in France. On 1 March 1689, O'Farrell was commissioned Colonel of the
Earl of Mars Regiment in place of
Thomas Buchan, a Scottish Catholic who had remained loyal to James. It reached
Flanders in time to fight at the
Battle of Walcourt in August 1689 and is listed in the 1691 return as a "
Fusilier" regiment, a designation reserved for elite units. O'Farrell was captured by a French patrol in July 1692 but was quickly released since he was present at the
Battle of Steenkerque in August and also fought at
Landen in 1693. However, in July 1695 as commander of the garrison at
Deinze he surrendered it to the French without resistance, the town of
Diksmuide doing the same. Prisoners were normally exchanged as soon as possible but the French retained the 6,000 - 7,000 troops captured at Diksmuide and Deinze due to a dispute over the terms of their surrender. Shortage of manpower was becoming an issue and the French used a various methods to encourage prisoners to enlist in their army, including moving officers and men into prison cells; O'Farrell wrote to a friend on 5 September complaining of this treatment. William dealt harshly with the officers from Deinze and Diksmuide; Ellenberg was executed in Ghent on 30 November, O'Farrell dismissed and those who signed the surrender documents court-martialled. Most were quickly reinstated, including O'Farrell who was reappointed in 1696, although he lost command of his regiment. During the
War of the Spanish Succession in January 1704, he was promoted
Major General, and according to
Parliamentary records was one of a number of general officers assigned to the army of
John V of Portugal, a position he retained until December 1708. He died in 1712. ==References==