 In early 1642, at the age of about sixty-two, her castle of Geashill was besieged by a force of insurgents from the O'Dempsey clan; she managed to hold out against them until October 1642.Her defence has been described as having been the "most spirited episode in the history of the Irish Rebellion of 1641".Lettice was born in about 1580, the only child and heir of Gerald FitzGerald, Lord Offaly by Catherine Knollys, who was a younger daughter of Catherine Carey and Sir Francis Knollys.