John was born about 1635[3] probably at Dromana, the only son of Gerald FitzGerald and his wife Mabel Digby.
He died in 1643 in his early thirties, probably killed in action while fighting the Confederates.
John and Katherine had an only daughter: FitzGerald's first wife died on 22 August 1660.
[10] FitzGerald married secondly Helen, daughter of Donough MacCarty, 1st Earl of Clancarty.
[12] When Charles II summoned the Irish Parliament of 1661–1666, FitzGerald stood for Dungarvan Bourough and was elected[13] as one of its two representatives.