Cosby, Francis (1510–80), soldier and planter, was the second son of John Cosby of Great Leak in Nottinghamshire and his wife Mabel Agard. About 1530 he married Lady Mary Seymour, daughter of the 1st duke of Somerset, with whom he had three sons; he was related to Sir Henry Sidney (qv) by marriage. He became a soldier, serving in the English forces in the Low Countries. In 1535 he arrived in Ireland in the retinue of Leonard Grey (qv), who was appointed lord deputy at the beginning of 1536. He was granted the manor of Kildare in 1536, where he resided. On 26 December 1537 he received a twenty-one-year lease on the lands of the dissolved nunnery of Hogges, which were located on the strategically important western fringes of the Pale, exposed to attack from the Gaelic clans of Laois and Offaly.
From about the mid-1540s Cosby fought in a number of campaigns against the O'Connor and O'More lords of the midlands. In December 1550 he received a lease of land under an abortive scheme for a plantation of the midlands, and the following year was granted a nine-year lease on Vicarstown in Co. Kildare. In 1554 Cosby surrendered the manor of Kildare to the recently restored 11th earl of Kildare, Gerald Fitzgerald (qv), and about this time appears to have moved to Monasterevin, Co. Kildare, where he was constable by 1555. On 13 July 1558 he was appointed general of the kerne, a troop of Irish light infantry, and thus acquired a constant retinue of thirty-two men. He served as sheriff of Kildare in 1558–60.
The restoration of the earldom of Kildare limited Cosby's influence in that county and his military duties meant that he was often stationed at Maryborough in Laois, where proposed plantation schemes presented more promising opportunities. In June 1557 the Irish parliament's decision to confiscate all land in Laois and Offaly prompted a period of bitter fighting between the native occupants and royal forces. The O'Mores nearly captured Maryborough in June 1558, but Cosby led a sudden counter-attack by the garrison and drove the besiegers away. In 1563 the government granted lands in Laois to English planters and some existing Irish occupiers, and he received about 2,000 acres at Stradbally. The O'Mores who did not receive lands went to war with the settlers. In recognition of Cosby's efforts against the rebels, he was transferred from the constableship of Monasterevin to that of Maryborough and in April 1566 was made seneschal of Laois, where he was also sheriff in 1564. About this time, he moved his residence to Stradbally. By 1563 he had divorced his first wife and married Elizabeth Palmer, with whom he had one daughter.
In the disordered conditions prevailing in the midlands, Cosby and other captains enjoyed free rein to terrorise the populace and to profit from the lax controls over military expenses. Cosby illegally accepted Irish tenants on his lands, levied black rents on English and Irish landholders, and cooperated with known rebels for his private gain. As English settlers quit the midlands he was able to buy their land cheaply, and by 1571 had doubled his holdings in Laois to over 4,000 acres. A commission in 1565 implicated Cosby and the others in the corrupt management of military funds, but no action was taken against him.
During the late 1560s Cosby cooperated with Rory Oge O'More (qv), a leading member of the McRory branch of the O'Mores, in running protection rackets that benefited them both. In May 1570 Sir Henry Sidney, lord deputy of Ireland and Cosby's brother-in-law by his first marriage, executed two of Rory Oge's cousins; only Cosby's influence saved Rory Oge from meeting a similar fate. This intervention was the worst mistake of Cosby's career for, in a fury over the death of his kinsmen, Rory Oge rallied the dispossessed O'Mores and O'Connors against the crown. An increasingly destructive war raged intermittently between the former allies for eight years. Both sides slaughtered women, children, and the elderly. The chaos in the midlands led in 1573 to the condemnation of Cosby by government officials for his past support of Rory Oge and more generally for his conduct as a planter and military official. In 1573–4 the queen twice ordered his dismissal from his commands, but surprisingly he remained in place. The reappointment of Sidney as lord deputy in 1575 eased the political pressure on Cosby and the two men soon resumed their pursuit of O'More.
With government forces closing in, O'More treacherously captured Cosby's son Alexander at a parley in 1577, though royal forces later rescued him. Exasperated by Rory Oge's continued survival, Cosby resorted to more extreme measures. In March 1578 he summoned a number of O'More's kinsmen to a parley at Mullaghmast in Laois. About seventy of them arrived unarmed and were massacred by Cosby's men. Cosby gained lasting notoriety for the slaughter, the memory of which was kept alive for centuries by local folklore and by nationalist historians. More immediately, the O'More resistance collapsed and Rory Oge was killed that June.
In his capacity as general of the kerne, Cosby accompanied the lord deputy, Arthur Grey, 14th Baron Grey of Wilton (qv), on campaign in Wicklow against rebel forces in August 1580. Despite Cosby's protests, Grey ordered his army into the valley of Glenmalure to attack the rebels there on 25 August 1580. Not only was Cosby concerned about the unfavourable terrain, but he also appears to have doubted the loyalty of his kerne, with whom he was unfamiliar, as the men had only lately been hired from Connacht. Nonetheless, he led the disastrous English advance into Glenmalure and was killed early in the ensuing battle, perhaps by his own kerne, many of whom joined the rebels. Local tradition relates that the rebel commander Fiach MacHugh O'Byrne (qv), who was Rory Oge's brother-in-law and whose sister had been killed by Cosby's men, carried Cosby's body away as a trophy of war and buried him in the hills above Glenmalure at Black Knobs. Cosby was succeeded by his eldest son, Alexander, who was killed, along with his son Francis, by rebel forces led by Uaithne O'More (qv), son of Rory Oge, at the battle of Stradbally bridge on 19 May 1596.