Francis Fergus O’Farrell was an Irish soldier of the seventeenth century. O’Farrell was originally from County Longford, but settled in the Dutch Republic where he married and had children. O'Farrell joined the Dutch Army of William III. O’Farrell was commissioned into the English Army as a Colonel following the Glorious Revolution of 1688. He served in Ireland under William during the War of the Two Kings which led to the defeat of the Jacobites under James II.
O’Farrell subsequently served in Flanders during the Nine Years' War, holding the rank of Brigadier. He was the commander of the garrison at Deinze when it was forced to surrender to the French, for which he was cashiered but later reinstated. By 1706 he had reached the rank of Major General after service with English forces in Portugal during the War of the Spanish Succession.