Young Lord Kildare became the sole male representative of the Kildare Geraldines at the age of twelve, after his half-brother, Silken Thomas, the tenth earl, was executed at Tyburn in February 1537 with five of his uncles. He spent the next few years on the run in Ireland and spent some time in Tír Chonaill (now mainly County Donegal) in Ulster, under the guardianship of his aunt, Lady Eleanor McCarthy, the wife of Manus O'Donnell, "An Ó Domhnaill". The short-lived Geraldine League, a federation including the O'Neills, the O'Donnells, the O'Briens of Thomond, and other powerful Irish clans related to the Geraldines through marriages, formed around FitzGerald's claim to the Earldom of Kildare. The League came to nothing, however, after the principal members were badly defeated in modern-day County Monaghan following a raid into The Pale in August 1539.