As Sheriff of Cork, Grenville witnessed the rebellion in which Fitzmaurice, along with the Earl of Clancar, James Fitzedmund Fitzgerald (the Seneschal of Imokilly); Edmund Fitzgibbon (the White Knight); and others, attacked Tracton. They overcame the English defence with pickaxes and killed nearly the entire garrison. The three surviving English soldiers were hanged the next day by the Irish. Fitzmaurice threatened the imminent arrival of Spanish forces. Having robbed the citizens of Cork, he boasted that he could also take the artillery of the city of Youghal.