Because only a bit more than a year later, on 28 October 1546, when John was about six, his father suddenly died in London having been poisoned during a banquet at Ely House,[9] probably at the instigation of Anthony St Leger, who was Lord Deputy of Ireland and a political opponent.

John and Katherine had four children, two sons: —and two daughters; Much of Kilcash's life was taken up with a fierce feud his family had with the Earls of Desmond.

Despite their enmity, these two families were both more or less Gaelicized Old English and had intermarried many times; the last such marriage having been that of Kilcash's parents as his mother was a Desmond FitzGerald.

[16] He was unable to participate in the private Battle of Affane, which was fought on 8 February 1565, only a bit more than a month after his mother's death.

[17] His stepfather, Gerald FitzGerald, 14th Earl of Desmond was taken prisoner in the battle[18] after Kilcash's brother Edmund had shot him into the hip with his pistol.

That meant that Edmund ceased to be Ormond's heir presumptive and John, the next brother, took his place but only for about a month as he died on 10 May 1570.

James fitz Maurice FitzGerald surrendered on 23 February 1573 and Gerald followed in September ending the first Desmond rebellion.