On his grandfather's death, Devereux became on 27 September 1558 The 2nd Viscount Hereford and 10th Baron Ferrers of Chartley. He was entrusted with joint custody of Mary, Queen of Scots, in 1568, and appointed Lord Lieutenant of Staffordshire in 1569. Lord Hereford, as he was now, provided signal service in suppressing the Northern Rebellion of 1569, serving as high marshal of the field under The 3rd Earl of Warwick and Lord Clinton. For his zeal in the service of Queen Elizabeth I on this and other occasions, he was made a Knight of the Garter on 17 June 1572 and was created Earl of Essex and Ewe, and Viscount Bourchier, on 4 May 1572. Eager to give proof of "his good devotion to employ himself in the service of her Majesty," Lord Essex, as he was now, offered on certain conditions to subdue or colonise, at his own expense, a portion of the Irish province of Ulster. At that time, Ulster, in the north of Ireland, was completely under the dominion of the O'Neills, led by Sir Brian MacPhelim and Sir Turlough Luineach, and of the Scots led by Sorley Boy MacDonnell. His offer, with certain modifications, was accepted. He set sail for Ireland from Liverpool in August 1573, accompanied by a number of earls, knights and gentlemen, and with a force of about 1,200 men. His enterprise had an inauspicious beginning; a storm dispersed his fleet and drove some of his vessels as far as Cork and the Isle of Man. His forces did not all reach the place of rendezvous till late in the autumn, and he was compelled to entrench himself at Belfast for the winter. Here his troops were diminished by sickness, famine and desertion to not much more than 200 men. Intrigues of various sorts and fighting of a guerilla type followed, and Essex had difficulties both with his deputy Fitzwilliam and with the Queen. He was in dire straits, and his offensive movements in the east of Ulster took the form of raids and brutal massacres among the O'Neills. In October 1574, he treacherously captured MacPhelim at a conference in Belfast, and after slaughtering his attendants, had MacPhelim, his wife and brother executed at Dublin. He arrested William Piers, who had been active in driving the Scots out of Ulster, and accused him of passing military intelligence to Sir Brian mac Phelim O'Neill. Essex ordered Piers's arrest and detention in Carrickfergus Castle in December 1574, but Piers was freed and he successfully executed Sir Brian mac Phelim O'Neill for treason. After encouraging Lord Essex to prepare to attack the Irish chief Sir Turlough Luineach O'Neill, apparently at the instigation of The 1st Earl of Leicester, the Queen suddenly commanded him to "break off his enterprise." However, she left him a certain discretionary power, and he took advantage of that to defeat Sir Turlough Luineach and chastise County Antrim. He also massacred several hundreds of Sorley Boy's following, chiefly women and children, who had hidden in the caves of Rathlin Island in the face of an amphibious assault led by Francis Drake and Sir John Norreys. He returned to England at the end of 1575, resolved "to live henceforth an untroubled life." He was, however, persuaded to accept the offer of the Queen to make him Earl Marshal of Ireland. He arrived in Dublin in September 1576, but fell ill at the banquet given in his honour at Dublin Castle, and died three weeks later, probably of dysentery. It was suspected that he had been poisoned at the behest of Lord Leicester, who married his widow two years later. A post-mortem was carried out and concluded that Essex had died of natural causes. He was succeeded in the Earldom of Essex by his son Robert.
