He was active in fighting on the edge of the English Pale, and was commended by the Lord Deputies Edward Bellingham and Thomas Radclyffe, 3rd Earl of Sussex.
[2] He helped to massacre, although the degree of his responsibility is not clear, many of the O'Mores at Mullaghmast, near Athy, who had been summoned to the fortress on avowedly peaceful business.
The murder of Rory in the following year relieved Cosby; but the outbreak of the Desmond rebellion in 1580 saw him killed by the rebels at the battle of Glenmalure, 25 August 1580.
Alexander succeeded to the estates, received additional grants in Queen's County, and was, with his son Francis, killed at the battle of Stradbally Bridge on 18 May 1596.
Arnold, Francis Cosby's second son, served under Robert Dudley, 1st Earl of Leicester in the Low Countries.